Question

Lisa invests $4,000 in two types of bonds: X, which offers a 10% return, and Y, which offers a 6% return. If the combined return on the investment is $300, how much did Lisa invest at the rate of 6%?

Answer 1

the two bonds quantity add up to $4,000
so
\(\bf \color{brown}{ x + y = 4,000}\)

one yield 10% and the other 6% respectively, so
0.10x and 0.06y

their return combined gave $300
so \(\bf \color{brown}{ 0.10x+0.06y = 300}\)

so you have a system of equations, you can just solve by substitution