Rob is a high school student taking both algebra and geometry. Last night he had 27 homework problems combined from the two classes, and he took a total of 90 minutes to do them. If each algebra problem took him 2 minutes and each geometry problem took him 5 minutes, how many algebra problems did he have for homework?

Let a be the number of algebra problems and g the number of geometry problems. The total number gives a + g = 27, and the time gives 2a + 5g = 90. Substituting a = 27 − g into the time equation gives 54 + 3g = 90, so g = 12 and a = 15. Rob had 15 algebra problems, choice D.

AI-written and checked against the verified answer.