To solve this, we need to add the 1/5 of the ice cream to the 2/3 left of ice cream to find out how much her sister ate.
First, we need a common denominator. We can multiply 3 by 5 to get a common denominator easiest, since they are both multiples of it.
3*5=15
Now multiply the numerators by the amount the denominators were multiplied by.
1*3=3 so 3/15
2*5=10 so 10/15
Now, add them together.
10/15+3/15= 13/15.
Finally, subtract that amount from the original full amount of ice cream (15/15) to get the amount her sister ate, since that is the only unknown value left.
15/15-13/15=2/15.
Because 15 isn’t divisible by 2, we can’t simplify this fraction anymore.
Her sister ate 2/15 of a gallon of ice cream.
Hope this helps!