Fewer means 'less than', and that always means subtract. In this case the problem is asking for the solution to 4/5 - 2/5. Whenever you are adding or subtracting fractions the fractions must always have the same denominator (the bottom number). This denominator will not change during the addition or subtraction process, just the top numbers (the numerators) have add or subtract to each other. Here is the problem and solution:
4/5 - 2/5 = 2/5
The process is just subtracting 4 from 2 and keeping the denominator (bottom number), 5..