it normally takes you 10 min to travel 5,0 mi to school along straight road. you leave home 15 minute before class begin. delay caused by a broken traffic light

the average speed for usual condition is =
= 5.0 mi / (10/60) h
= 30 mi/h

let us check whether in 15 minutes (with those condition) I am gonna be late or not...

the total time of my trip is:

a) in a delay traffic :

time = distance / speed

t = (2.0 mi) / (20 mi/h)
t = 0.1 h  -->> it means 1/10 * 60 = 6 minutes

so the elapsing time was 6 minutes here.

b) in a usual traffic
remember that the remaining distance is still = 5.0 mi - 2.0 = 3.0 mi

time = distance / speed

t = (3.0 mi) / (30 mi/h)
t = 0.1 h -->> it means 1/10 * 60 = 6 minutes
so the elapsing time was 6 minutes here

hence, the total time was = 6 min + 6 min = 12 minutes.

Yeeaayyyyyy !!!
I will be not late for class !
:D