You need to know the Time “T” (“how long does the car travel at the reduced speed). What do you know out of the equation above based on the word problem?

“reduces its speed by 15 miles per hour for the next 4 miles”

*From this you know that the distance “D” traveled is 4 miles.

*Now in order to solve the equation for the time, you will need to know the rate of speed “R”.

“A car travels at an average speed of 45 miles per hour …, then reduces its speed by 15 miles per hour”

Reduced speed is equal to 45mph – 15mph, or 30 mph.

Now go back to the original equation.

D=R*T

4 miles = 30 mph * T

4 / 30 = T

So, T = 4/30 of an hour.

4/30 hr x 60min/hr = 8 minutes