The simplest form of an interest equation is A = P(1+rt)
where A = the total amount of money at the end, P = the principal (or amount of money you started with), r = the rate in percent, and t = the time in years.
In this case, P = 15000, r = 0.03 (because 3% in decimal form is 0.03), and t = 1:
\(A=P(1+rt)\\A=15000(1+0.03*1)\\A=15000(1+0.03)\\A=15000(1.03)\\A=15450\)
So, after 1 year he will get $15450 back, making him $450 more.
