Let x represent the number of math problems that you completed.
Let y represent the number of biology problems that you completed
From the information given,
You completed a total of 15 problems. This means that
x + y = 15
If each math problem takes 5 minutes to complete, it means that the time to complete x math problems is 5 * x = 5x
If each biology problem takes 3 minutes to complete, it means that the time to complete y biology problems is 3 * y = 3y
If you spent 51 minutes working on your homework problems for math and
biology, it means that
5x + 3y = 51
Thus, the system of equations is
x + y = 15
5x + 3y = 51