Given
Peter invested some money at 6% annual interest, and Martha invested some at 12%. If their combined investment was $4,000 and their combined interest was $360, how much money (in dollars) did Martha invest?
Solution
Let Peter's investment be X
Let Martha's investment be Y

I will solve your system by substitution.
(You can also solve this system by elimination.)
Step 1

Step 2
Substitute for x in Equation (ii)

Step 3

The final answer
Martha's investment is $2000