After 5 years, the simple interest earned on an investment of $3000 was $645. find the interest rate.

The simple interest formula is: 
A=P(1+rt)
P is the principle amount so P = $3000
A is the total amount accumulated
r is the rate divided by 100
t is the time

When t = 5, A = 3 645
so,
3 645 = 3000(1+5r) - divide both sides by 3 000
1.215 = 1+5r - subtract 1 from both sides
0.215 = 5r - divide both sides by 5
0.043 = r
however, need to multiply r by 100 because the equation for r is R/100
so 0.43 x 100 is 4.3
The interest rate is 4.3% per year