In how many days can the work be completed by 10 women ?
I. 5 men can complete the work in 8 days
II. 6 men and 4 women together can complete the work in 5 days
III. One man and one women together can do thrice the work done by a women in one day.

Let the amount of work done by a woman in a day be W and let the amount of work done by a man in a day be M.
We need to find the value of W to be able to find the number of days it takes 10 women to finish the work.

Statement 1 implies that: $$5M = \frac{1}{8}$$
So, $$M=\frac{1}{40}$$

Statement 2 implies that: $$6M+4W = \frac{1}{5}$$

Statement 3 implies that $$M+W = 3W => M=2W$$

Clearly using just one statement, we can't find the value of W. But using any pair of statements, we can find the value of W as shown below.

Using Statement 1 and 2: $$M=\frac{1}{40}$$ and $$6M+4W = \frac{1}{5}$$
So, $$\frac{6}{40}+4W=\frac{1}{5}$$
So, $$4W = \frac{1}{20}$$ or $$W=\frac{1}{80}$$
So, 1 woman can do the work in 80 days and 10 women can do the work in 8 days.

Using Statement 1 and 3: $$M=\frac{1}{40}$$ and $$M=2W$$
So, $$2W=\frac{1}{40}$$ or $$W=\frac{1}{80}$$
So, 1 woman can do the work in 80 days and 10 women can do the work in 8 days.

Using Statement 2 and 3: $$M=2W$$ and $$6M+4W = \frac{1}{5}$$
So, $$12W+4W = \frac{1}{5}$$ or $$16W=\frac{1}{5}$$
So, $$W=\frac{1}{80}$$
So, 1 woman can do the work in 80 days and 10 women can do the work in 8 days.

As can be seen above, the question can be answered using any two statements of the three.