The formula for simple interest is I = prt, where I is the amount of interest, p is the principal invested, r is the rate as a decimal number and t is the amount of time. Substituting our given information we have
128 = p(0.016)(2) ----- 1.6% = 1.6/100 = 0.016
128 = p(0.032)
Divide both sides by 0.032:
128/0.032 = p(0.032)/0.032
4000 = p
He needs to invest $4000.
