Question 211047
Let x be the amount of money invested at 7%. 
Then the amount invested at 4% is 2100 - x.

The interest earned from the money invested at 7% is x*7%
The interest earned from the money invested at 4% is (2100 - x)*4%

As the total interest earned from the two investments is given as $120, we have:
x*7% + (2100 - x)*4% = 120

Solving for x, we get
7x + 4(2100 - x) = 12000  (multiply both sides by 100)
7x + 8400 - 4x = 12000
3x + 8400 = 12000
3x = 3600
x = 1200

So $1200 is invested at 7%, and $2100 - $1200 = $900 is invested at 4%.