Add the expense percentages.
20%+23%+42%= 85%
Subtract the percentage you found from 100%.
100%-85%=15%
This percentage is the amount of money the family saves monthly, in this case, $360.
To find how much money the family makes monthly, divide 100 by 15 and multiply that by $360.
$${\frac{{\mathtt{100}}}{{\mathtt{15}}}}{\mathtt{\,\times\,}}{\mathtt{360}} = {\mathtt{2\,400}}$$
The amount of money the family earns in a month is $2400.