Answer:
They can make 9000 crayons in 15 minutes.
Explanation:
Figure out the ratio:

Simplify it to get the total number of crayons that they can make in one minute.
2400/4 = 600
They can make 600 crayons per minute.
Now, you should have a proportion.

Now, multiply the denominator and numerator by 15 because whatever you do to the denominator you have to do to the numerator.
You should now have your answer.

Crayola can make 9000 crayons in 15 minutes.