If a man and a half can eat a cake and a half in a minute

  1. If a man and a half can eat a cake and a half in a minute and a half, allowing the same situation, how many men would it take to eat 60 cakes in 30 minutes?

    Btw, I'm getting these off an app on my phone.
  2. jcsd
  3. DaveC426913

    DaveC426913 16,243
    Gold Member

    Re: Brainteaser

    One cake per minute per man.

    So, 2 men.

    I want half the prize money.
  4. Re: Brainteaser

    Not quite-- If 1 man could eat 1 cake in 1 minute, then the one man in the example would've eaten 1 cake in 1 minute, while the other half-a-man would've eaten his half of a cake in 1 minute. So if it were 1 cake per minute per man, it'd be gone in 1 minute, rather than a minute and a half.

    If R is the number of cakes per minute that 1 man can eat, then:

    1.5 men * R * 1.5 minutes = 1.5 cakes
    1.5 men * R minutes = 1 cakes
    R = 1 cakes / (1.5 men minutes) = 2/3 cakes/men minutes

    Hence, we now want:

    Q men * 2/3 cakes/men minutes * 30 minutes = 60 cakes
    Q men * 2/3 = 2 men
    Q men = 3 men
    Q = 3

  5. DaveC426913

    DaveC426913 16,243
    Gold Member

    Re: Brainteaser

    : faceplant :

    I misread. I thought it was 1,1.5,1.5 not 1.5, 1.5, 1.5.
  6. Re: Brainteaser

    I have another one:

    If you have a sphere and you remove a cylinder with a length of six inches from it (assume that the cylinder is arranged in such a way so as to go through the maximum amount of the sphere), what is the new volume?
  7. Re: Brainteaser

    Maybe I'm missing something-- I get:


    (where R is the radius of the sphere.) I'm guessing that some factors are supposed to cancel out nicely?

  8. Re: Brainteaser

    The answer is independent of radius because as the radius of the sphere increases the radius of the cylinder also increases, so the volume percentage remains the same. Kind of like why rain water depth is independent of the the volume used to measure it
    Last edited: Oct 18, 2009
  9. Re: Brainteaser

    I think there's some sort of stipulation that's being implied. Here's what I get using two extremely different values for the radius of the sphere (see attachment). Am I missing some way of arranging the cylinder such that it takes up more volume?


    Attached Files:

    Last edited: Oct 18, 2009
  10. Re: Brainteaser

    It looks like you've set the length to be >6 in the larger sphere, close to the diameter at 18 (?). Length is specified at 6, radius can vary
  11. Re: Brainteaser

    What about differentiating the expression for the volume difference to find the value of radius\diameter that minimises it?

    So I get for the volume difference V = 4/3*pi*(d/2)^3 - 6*(d^2-36)*pi/4 where d is the diameter of the sphere, which upon differentiating and setting to zero gives either a diameter of 9 or -15. Using 9 gives the difference as 54*pi.
    Last edited: Oct 19, 2009
  12. Re: Brainteaser

    are we allowed complex solutions?
  13. Re: Brainteaser

    I'm not sure about complex solutions, but I just realised I factorised the quadratic wrong. I should have got a diameter of ~7.82 and a volume difference of ~43*pi.
    Last edited: Oct 19, 2009
  14. Re: Brainteaser

    Okay I've completely messed up that differentiation, I should've got d = 6 or 0 and that would maximise the volume difference so no good.
    Last edited: Oct 19, 2009
  15. Re: Brainteaser

    I've done nothing of the sort. The cylinder in the larger sphere resembles a disc: it's 6" tall, and extremely wide (big radius). The cylinder in the smaller sphere resembles a rod: it's also 6" tall, but is extremely narrow, in order to fit within the sphere. In both images, the circle is the sphere, and the rectangle is a sideways view of the cylinder, with the left and right sides being the straight "length" of the cylinder, and the top and bottom sides representing the diameter of the cylinder.

  16. Re: Brainteaser

    yes, I see, I didn't look carefully enough. I agree with your earlier analysis...I was remembering a strikingly similar problem in which the answer was a constant, not related to either radius
  17. Re: Brainteaser

    So to repeat arithmetix' question, are we allowed complex solutions? I get two possible complex ones: (45+i*9*(27)^1/2)*pi or (45-i*9*(27)^1/2)*pi (assuming I haven't made anymore silly mistakes, which is a big assumption :redface:).
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