For interest questions, Notice the following three pieces of the puzzle:
1) Interest rate - usually given as a %
2) Time - How long the interest is calculated for.
3) Compounded - Annually, semi-annually (every six months), quarterly, monthly.
Simple Interest formula = Principal × Rate × Time
Use simple interest in the following cases:
1) The question explicitly use the phrase "simple interest"
2) The calculation time for the interest is shorter than the stated interest period. e.g. 12% annual interest calculated after a 6-month time period.