After 9 months, the simple interest earned on an investment of $1200 is $72. find the interest rate.

Use the formula i=prt, where i is the interest earned, p is the principal, r is the rate (as a decimal fraction) and t is the elapsed time, in years.

Here i = $72 = $1200 r (9/12)                    (9 months is 9/12, or 3/4, of 1 year)
Reducing, 
              $72 = $900r
                                          Solving for r, r=0.08, or 8 percent per year.