Subtract the number she completed from what she had left to paint originally.
The fractions have different denominators so first step is to rewrite the fractions with a common denominator.
23/25 can be rewritten as 92/100
Now you have 78 21/100 - 34 92/100
Because 21/100 is smaller than 92/100 subtract 1 whole number from 78 and rewrite 1 as 100/100 and add that to the fraction.
78 21/100 becomes 77 121/100
Now subtract :
77 121/100 - 34 92/100
77-34 = 43
And 121/100 - 92/100 = 29/100
Combine to get 43 and 29/100
