How much was invested at each rate?

Question

the computer club invested $2200, part at 4.5% interest and the rest at 7.2%. the total annual interest earned was $144. How much was invested at each rate?

Answer 1

Let Part = x invested at 4.5%
and
Let invested at 7% = 2200 -x 
So
(x)(0.045) + (2200-x)(0.07) = 144

0.045x -0.07x = 144 - 0.07*2200 = 144 - 154

-0.025x = - 10

x = 10/0.025 = $ 400
7%= $ 1800 

Answer 2

Let the amount invested at 4.5% be x
The amount invested at 7.2% will thus be 2200 - x

The annual interest earned from the 4.5% will be 0.045x
The annual interest earned from the 7.2% will be 0.072(2200 - x)

Since the total annual interest is $144, we have:

144 = 0.045x + 0.072(2200 - x)
144 = 0.045x + 158.4 - 0.072x
144 = 158.4 - 0.027x
0.027x = 158.4 - 144
0.027x = 14.4
x = 14.4 / 0.027
x = 1600 / 3

2200 - x = 5000 / 3

Hence, $(1600 / 3) is invested at 4.5% and $(5000 / 3) is invested at 7.2%