Hi Joanna. I'll show you how to solve this type of problem by using my own example:

Q. It normally takes four volunteers five days to build an outdoor hockey rink. How many people need to work together to get the job done in two days?

A. The key is to understand the amount of "effort" is required to do the job, regardless of how many people are working or how long the project lasts. The total effort is the number of people times the amount of effort each person contributes. So in my case, it takes 4x5 = 20 units of effort. We could call these worker-days. So to answer the question, we assume that the job will always need 20 units of effort to get done. If you need to get that done in 2 days, you need 10 units of effort each day, so you need 10 volunteers working together.

With mathematics, we write it like this: 4 x 5 / 2 = 10.

Now you can try your question using this method.

Cheers,
Stephen La Rocque.