A student spends time each evening on two subjects: reading and writing. The equation above models the total number of minutes the student spends on these two subjects, where is the number of minutes spent reading and is the number of minutes spent writing. In the equation, what does the number 90 represent?
The number of minutes spent reading each evening
The number of minutes spent writing each evening
The total number of minutes the student spends on reading and writing each evening
The ratio of minutes spent reading to minutes spent writing
