We know that at the beginning of the day we have 7/9 of a bushel of plums and that after a while there are 1/3 of a bushel of plums, so to calculate how many were sold we have to subtract the bushel of plums that were left from the initial ones
7/9 - 1/3
if we want to do this subtraction we have to find a fraction equivalent to 1/3 that can be subtracted from 7/9
1/3 = (1 * 3) / (3 * 3) = 3/9
now we are just the top and leave the denominator the same as before
