Ihave three times as much money as does my brother. our combined total is $42. how much does each of us have?

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

You have $31.5 and your brother has $10.5