Gilda walks to the train station. If she walks at the rate of 3 mph, she misses her train by 7 minutes. However, if she walks at the rate of 4 mph, she reaches the station 5 minutes before the arrival of the train. Find the distance Gilda walks to the station.

My answer:
We'll represent the distance as d and the time as t.
d= 3(t+7) because Gilda is 7 minutes late when travelling 3 mph
d= 4(t-5) because Gilda is 5 minutes early at 4 mph

d= 3t + 21
d= 4t -20
3t +21 =4t -20
41=t
d= 3(41) +21= 144
The distance is 144 miles.

My teacher said: " Did you check all of your units to make sure that you are using all the same type of measurements?"..