First of all we had to analyze the given which is 20 1/2 kgs of fish which is the total fish he had.6 1/4 kgs she sold first and another 12 1/4 kgs
The formula for this is:
20 1/2 - (6 1/4 + 12 1/4) = the kilograms of fish unsold
Symplify the equation and you will get:
20 1/2- 18 1/2 = kgs of fish unsold
And we finally got our answer which is 2 kgs of fish unsold