Explanation:

Total work = man/woman time
Work is same so,
16m $$\times 8 = (8m + 9w) \times 10
128m = 80m + 90w
48m = 90w

=8m = 15w
Let the 20 women complete work in days so,
Total work = 20w d
=(8m + 9w) \times 10 = 20w d
=(15w + 9w) \times 10 = 20w d
=24w \times 10 = 20w d
=d = 12 days
Twenty women finish the same work in 12 days.