Hod do I solve this type of question?

Question

Susan placed $5,700 into three investments: Account A paid 10% simple interest, Account B paid 12% simple interest, and Account C paid 18% simple interest. The total interest earned after one year is $792. If the amount of money placed in Account C is $400 more than the amount placed in Account B, find the amount of each investment in dollars.

Answer 1

.10A +.12B +.18C = 792

B=400+C

A+B+C =5700

A +400+C +C =5700

A=5700-2C-400= 5300-2C

A=5300-2C

.1(5300-2C) +.12(400+C) +.18C = 792

.-20C +.12C +.18C = 792 -530-48

..1C = 214

C = $2,140

B=400+C = $2,540 invested in B

A=5700-2140-2540 = $1020 invested in A

check the answers:

.1(1020)+.12(2540)+.18(2140)

=102 + 304.8 + 385.2

=792