A student is saving money for a trip. They earn $12 per hour for tutoring, $15 per hour for dog walking, and $10 per hour for babysitting. The student's goal is to earn a total of $600. If the student spends an equal number of hours tutoring and babysitting, and $t$ represents the hours spent tutoring, $d$ represents the hours spent dog walking, and $b$ represents the hours spent babysitting, which of the following expresses $d$ in terms of $t$?