When my daughter got 7 years old, she asked me, how to answer the problem in time calculation easier.
She got some problem about starting time, duration and arriving time. She asked me, how to calculate it easily...
I just say, you can use most basic math operation, ( + ) or ( - )
Here :
A + B = C
B = C - A
A : Starting Time
B : Duration
C : End Time/Arriving Time
To make easier, use 24 H clock time, and you can use all above simple formula
After she understand, she told to her teacher about the formula, and the teacher said, perfect...smart...thank you....