Lisa has a total college account of $120,000. $50,000 of this is an account that pays 3.35% interest. What interest rate would she need on the rest in order to get $6,000 per year in interest all together?
First, calculate what the interest will be on $50,000 during one year, at interest rate 3.35%
Subtract that interest amount from $6,000 to see what interest the other account would need to yield; this would be on $70,000 (remaining principal). So you only have to solve I=PRT where I was just calculated by subtraction, T = 1 year, and P = 70,000.
The only unknown is the interest rate, R. Now go to it!.