You deposited Php30,000 in a savings account that 9% simple interest per year. How many years will it take double your balance? If instead you deposit the Php30, 000 in another savings account that earns 8% interest compound yearly, how many years will it take to double your balance?
What formula should I use in each situation?
simple interest doubles in x years if 1.09x = 2
compound interest doubles when 1.09^x = 2
Which formula? You probably have only learned two or three formulas so far; it can't be that hard to choose the one you want.