Recall that to compute for the new amount of the original sum at a compound interest is
A(t) = P(1 + r/n)^(nt)
where P is the original amount, r is the interest rate, n is the number of times P is compounded each year, and t is the number of year.
With this, the total amount owed by Sam in five full years becomes [(120)(1 + 0.30/12)^60 ]. The total interest is the difference between the original amount owed and the amount that needs to be paid over time. Thus, total interest is [(120)(1 + 0.30/12)^60 ] - 120 = 407.97.
Therefore, the total interest is $407.97.
