I have a question asked in a previous exam, where to find simple interest for y years at x per annum instead of x% per annum. If I use (p * 3 * 40)/100 to compute the value, the answer gets wrong. The answer is given in a solution where the formula taken as (p * 3 *4)/100 without any explanation. The question is:
The simple interest on a certain sum of money for 3 years at 40 per annum is half the compound interest on $2000 for 2 years at 10% per annum. What is the sum invested on simple interest?
What formula should I use? Also what is the difference between x% per annum and x per annum with this regard.