For an instance, i stands for the money that you have, and b stands for the money that brother has. An equation system based on the problem would be i + b = 42 (equation 1) i = 3b (equation 2)
Use substitution method to solve the problem substitute 3b into i in the equation 1 i + b = 42 3b + b = 42 4b = 42 b = 42/4 b = 10.5
substitute the value of b into the equation 1 i = 3b i = 3(10.5) i = 31.5