4 kitchen assistants can prepare 10 kg of vegatables in 45 minutes. How long would it take 15 assistants to prepare the same amount?

Seeing it at first, you might think about using simple proportions, but logic tells you that they should complete it in a lower period.

This is a problem which requires inverse proportions.

It is mainly in the form y = k\(\frac1x\)

where y is the time and x is the number of assistants.

Using the first piece of information, you can obtain the value of the constant k.

Then, you use that constant k with the new asked number of assistants and solve for x, the time that they take. What do you get? :)