If we want to know how much she spends on food and rent, we need to add the two amounts together, so we have
3/7 + 1/7 = 4/7. Remember that when adding fractions, we need a common denominator to start, and then we add the numerator and keep the denominator the same. All of her money can be represented as 7/7, since we've been splitting
it into seven parts, and we want all of the parts. So take how much she started with and subtract the food and the rent, so 7/7 - 4/7 = 3/7. The same process applies to subtracting fractions as it does with adding fractions.