To answer this question you would start with 1 whole piece and break it into 4 equal pieces. Each piece would be 1/4 of the original.
If you used 3 of these, you are left with 1/4.
Out of the 1/4 leftover, you create 5 equal-sized pieces.
1/4 divided by 5.
1/4 x 1/5 = 1/20 of the original board for each.