A student invested $4200 in certificate of deposit, one at 2% and the other at 3%. Find the amount invested at each rate of the first year combined interest income from the two investments was $102.

The traditional way:

Let x=amount invested at 2%, then (4200-x) = amount invested at 3%

We're given

0.02x+0.03 (4200-x) = 102

Solve for x:

0.03x-0.02x=126-102

0.01x=24 = > x=2400

Hence 2400 invested at 2% and 1800 invested at 3%

Alternative:

4200 at 2%=84 4200 at 3%=126

Amt at 3%:Amt at 2%=102-84:126:102=18:24 = (18/42) * 4200: (24/42) * 4200=1800:2400