Question

Darren teaches a class of 25 students. He assigns homework 3 times a week, and each assignment consists of 12 problems. How many problems must Darren correct each week?

Answer 1

You could break this problem into steps:
1) calculate how many problems Darren assigns to EACH student each week: It's 3*12 problems per student.
2) Multiply that result by the number of students. The result is the number of problems Darren must correct each week.

Units of measurement can be very helpful in determining whether or not your answer makes sense.