one fifth of a plank is sawn off and there eights of the remaining peice is then thrown away. What fraction of the original plank remains?

Answer 1

after 1/5 gone, there are 4/5 remaining.
3/8 * 4/5 = 3/10
So, what remains is 1 - 1/5 - 3/10 = ______

Answer 2

1 - 1/5 = 5/5 - 1/5 = 4/5 remains.

4/5 - 3/8 = 32/40 - 15/40 = __ remains.

Answer 3

Correction:
1 - 1/5 = 5/5 - 1/5 = 4/5 remaining.
4/5 - 3/8*4/5 = 4/5 - 12/40 = 32/40 - 12/40 = 20/40 = 1/2 remains.