Well first you need to find the LCM (least common multiple) of 4 and 6 (the denominators) so that you can subtract them from each other.
4: 4,8,12,16,20
6: 6, 12, 18, 24
The LCM is 12.
Now we can subtract the two mixed numbers by turning them into improper fractions.
5 3/12 -> 27/12
3 10/12 -> 42/12
42-27=15
The fraction is 15/12, which simplifies into 1 1/4.
She cut off 1 1/4 feet of the board.
