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:

So, after 1 year he will get $15450 back, making him $450 more.
Easy, showing work,
using the method FOIL (First,Outer,Inner, Last)
Take, (x+3)(x+5) and multiply,
First,
x*x=

Outer,
x*5=5x
Inner,
3*x=3x
Last,
3*5=15
Add the terms together,

+8x+15 (

+5x+3x+15)
Thus, your answer.
The answer is 30%.... because 50/65 is .3