Question

A college student earned $7200 during summer vacation working as a waiter in a popular restaurant. The students invested part of the money at 10% and the rest a 9%. If the student received a total of $680 in interest at the end of the year, how much was invested at 10%?

Answer 1

You could use systems of equations:
X = amount invested in 10%
Y = amount invested in 9%

X + Y = 7200
0.1X + 0.09Y = 680

Solving for Y @ 1st equation:
Y = 7200 - X
Substitute to 2nd equation:
0.1X + 0.09(7200 - X) = 680
0.01X = 32
X = 3200

Therefore, the amount invested @ 10% interest is $3200

Answer 2

$3200 at 10% and $4000 at 9%

7200 = a + b
680 = 0.10a +0.09b

then solve simultaneously