Answer to the Friday Puzzle….

44

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On Friday I posted this puzzle….

I have a cage in my garden and it contains only chickens and rabbits (I don’t really, but play along). There are 72 heads and 200 feet in the cage. How many chickens are there, and how many rabbits?

If you have not tried to solve it, have a go now.  For everyone else the answer is after the break.

There are 44 chickens and 28 rabbits in my cage.  Did you solve it? And if so, how?

I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle (UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.

44 comments on “Answer to the Friday Puzzle….

  1. Eddie says:

    I solved it by inverting a 2 by 2 matrix.

  2. du Lisoy says:

    X+Y = 72
    2X+4Y = 200
    Y being the number rabbits…

    • ChrisR says:

      and for those who want it spelled out (who can’t remember ‘simultaneous equations’)..
      X+Y=72 means 2X+2Y = 144
      so the difference between this and du Lisoy’s last equation is
      2Y on the left and 56 on the right, so there are 28 rabbits.

      X + 28 = 72 means X (chickens) = 44

      Given the numbers involved (and knowing you wouldn’t get an answer including 1/2 a rabbit) it was probably easy enough to do in one’s head, but there’s a choice between doing it on paper straight-away and knowing you’re on the right track, and the risk of starting off doing it in your head but having to resort to pen/paper if the numbers got awkward.

      I’m interested in seeing what short-cuts there may be (and not trial and error).

    • mgm75 says:

      (and knowing you wouldn’t get an answer including 1/2 a rabbit)

      Mine did because a blasted fox got in over night!

  3. Andrew says:

    If all 72 animals had two legs there would be 144 feet, so the other 56 feet must belong to the rabbits. Two extra feet per rabbit, means 28 rabbits.

  4. VicinCA says:

    x = # of rabbits (4 legs)
    y= # of chickens (2 legs)

    Use simultaneous equations to solve for the # of rabbits.
    x + y = 72
    4x + 2y =200

    4x + 2y = 200
    2x + 2y = 144

    Subtract the equations.
    2x = 56
    x = 28 rabbits

    72 – 28 = 44 chickens

  5. Geodetective says:

    72 heads and 100 pair of feet. That’s 28 pairs too many in case of only chickens, which makes 28 rabbits. 72 – 28 = 44 chickens. Did not need an (advanced) pen or paper.

  6. Stan says:

    The number of left feet is 100 and equals the number of chickens plus twice the number of rabbits.
    The number of heads is 72 and equals the number of chickens plus the number of rabbits.
    Thus the difference of 100 – 72 is the number of rabbits.

    I was able to do that without inverting a matrix or writing down two equations with two unknowns. (Actually I’ll have to admit the latter is how I did it on Friday).

  7. zippy says:

    72 heads is 72 animals. If they were all chickens that would be 144 feet, which is 56 too few. Therefore 28 of the chickens need to be replaced with rabbits, leaving 44 chickens.

    Interesting to see the split between people who thought it through simply like this and those who saw it in the form of pen and paper equations.

  8. Roland says:

    I had 16 chicken and 56 3-legged rabbits (their fourth leg was sold as lucky rabbit’s foot, the poor animals…).

    • ivan says:

      I accidentally solved that problem first too. Then I remembered rabbits have 2 more legs than chickens, and halved the 56 (=200-144). That’s why it took me 10 seconds instead of 3 seconds.

      I’m disappointed that Richard didn’t give the simple explanation why the answer is rabits = 100-72. Looks like he solved simultaneous equiations too.

  9. I found the same answer, but instead of using equations like the rest of you, I used an estimate. I asked: what if 100 of those feet belonged to chickens, and the other 100 to rabbits, how many heads would I have? The answer turns out is 75, which is only 3 off. Plussing and minussing lead me to the correct answer within seconds.

  10. pete says:

    What about myxomatosis?

  11. lindamp says:

    Assuming each chicken has 1 head and 2 feet, and each rabbit has 1 head and 4 feet:

    Each animal has at least 2 feet. 72 animals.
    72 * 2 = 144 feet

    200-144 = 56 “extra” feet

    56 extra feet = 28 rabbits with 4 feet.

    Therefore 28 rabbits and (72-28 =) 44 chickens.

    • Nat says:

      ha-ha- you assumed chickens and me rabbits as chicken it is a bird (i was taught like this in old school 20 years ago) and not the animal, so 72 x 4 = 288
      288-200=88
      88:2 (legs of chickens)= 44
      so 72 – 44-= 28
      well it took me some time

  12. Julia says:

    This puzzle was a revelation to me. I was on the phone (listening to the various options for buttons to press) when I read and decided to use algebra (same method as that used by du Lisoy and VicinCA) as I wouldn’t be able to concentrate hard enough for trial and error. If anyone had asked me what’s easier – algebra or trial and error – I always would have opted for trial and error. Turns out I was wrong.

  13. Duncan says:

    Some very elegant solutions, but apart from using matrices aren’t they all just simultaneous equations?

  14. issahamati says:

    wow

  15. L.Long says:

    But he said….
    ‘There are 72 heads and 200 feet in the cage.’
    So obviously they had a great BBQ and there are no chickens or rabbits left in the cage. So the answer is 0.

  16. Jaleel Hamid says:

    Got it… But it took me a few minutes!😉

  17. I did it through the “extra” legs method…. but then did it by simultaneous equation, except I solved the equations a slightly different way than the others shown

    c + r = 72
    2c + 4r = 200 which I auto simplify almost instantly by dividing by 2…

    c + 2r = 100

    isolate c…

    c = 100 – 2r

    plug it into the first as the value of c

    100 – 2r + r = 72

    100 – r = 72

    -r = 72 – 100

    -r = -28

    negatives cancel

    r = 28;

    • ChrisR says:

      isolating ‘r’ from the first two equations, by taking the first from the auto-simplified second, is a little simpler and is actually how the ‘elegant solvers’ did it in their heads – even if it wasn’t called simultaneous equations

  18. I used simultaneous equations…..at least 30years since I last did them!!! But totally with keithnicholas is using r & c instead of x & y. Teaching my son at the moment and using algebra that has more meaning….r=rabbit & c=chicken just brings maths and puzzles more to life and into the real world……or is that just me?

  19. Don’t Rabbits have 2 feet & 2 paws?

  20. Bill says:

    I multiplied the total num of heads with the feet of the rabbits and it was a remainder of 88 feet. So, I thought that 88 would be the feet of the chickens and therefore 44 heads.

  21. Ronda says:

    I used two equations
    72=C+R
    200=2C+4R or 100=(C+R)+R

    Then plugged the first into the second for 100=(72)+R, which gives 28 rabbits.

  22. Like nearly everyone else I just used algebra 2C + 4(72-C) = 200 where C= number of chickens.

  23. Anonymous says:

    x+y=72
    4x+2y=200
    Isolate y by subtractracting x.
    y=72-x
    4x+2(72-x)=200
    4x+144-2x=200
    Combine like terms
    4x-2x=2x
    2x+144=200
    subtract 144
    2x=56
    divide by 2
    x=28
    Then plug back into original equation.
    x+y=72
    28+y=72
    subtract 28
    y=44
    x=rabbits=28
    y=chickens=44

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