Question

Lisa can make a costume twice as fast as Justin can. If they both work together, they can make it in 8 hours. How long will it take Mary to mike the costume if he works alone?

Answer 1

First, I assume you mean "How long will it take Justin to make the costume if he works alone?"

So, here's how I thought about it.

Suppose a costume consists of n units of work.

So, together, Lisa (L units/hour) and Justin (J units/hour) can make n units in 8 hours:

L*8 + J*8 = n

We are given that L = 2J. Thus:

3J*8 = n => J = n/24

So, L = n/12.

Lisa by herself, then, would take L*h = n => n/12*h=n => 12/h=1 => h=12.

So, 12 hours.

Answer 2

ohh. i understand now. thanks for your help! @queelius

Answer 3

You're welcome.