Q:

Simple interest. Finding how much an investment must be out into an account from the start

so the question is: Investment account offers a simple interest rate of 6%. If a woman wants $31,000 after 4 years, how much must he put into the account at the start?

A:

Since this is a simple Interest (and not a compounded interest), after 4 years the amount she will get is 
  = Principal * (1 + (6 * 4)/100)
  = Principal * (1 + 24/100)
  = Principal * (100 + 24)/100
  = Principal * (124/100)

If she wants it to be $31,000
  $31,000 = Principal * (124/100)

Therefore, 
 Principal =  (31000 * 100)/124 = $25,000

