How much was invested at 3.75%?

Question

Walt made an extra $10,000 last year from a part-time job. He invested part of the money at 2% and the rest at 3.75%. He made a total of $270.00 in interest. How much was invested at 3.75%?

Answer 1

Let X be the amount invested at 2%

Let Y be the amount invested at 3.75%

A = interest made at 2%

B = interest made at 3.75%

You can derive 4 equations

1) A + B = $270

2) X + Y = $10,000

3) A = X * .02

4) B = Y * .0375

You have 4 equations with 4 unknowns.

Need to solve for Y

Solving Equation 1) for B give  B = 270 - A

Substituting that into equation 4) give

270 - A = Y * .0375

Putting Equation 3) into this for A give

270 - (X*.02) = Y * .0375

Solving equation X in equation 2) gives

X = 10000 - Y

Putting that into the previous equation gives

270 - (10000 - Y) *.02 = Y * .0375

Solving

270 - 200 + .02 * Y = .0375 * Y

70 = .0175 Y

Y = 4000

Checking the problem

X = 6000

 

A = 6000 * .02 = 120

B = 4000*.0375  = 150

A + B = $270

X + Y = $10,000