There is a total of 30 erasers
Multiply 20 by 5 and you get 100, please tell if this is what you wanted!
2.5 * 3.5 = 8.75 or 8 3/4 inches long
I believe it is option 2, sorry if incorrect
Firstly, we'll fix the postions where the women will be. We have forms to do that. So, we'll obtain a row like:
The n+1 spaces represented by the underline positions will receive the men of the row. Then,
Since there is no women sitting together, we must write that . It guarantees that there is at least one man between two consecutive women. We'll do some substitutions:
The equation (i) can be rewritten as:
We obtained a linear problem of non-negative integer solutions in (ii). The number of solutions to this type of problem are known:
[I can write the proof if you want]
Now, we just have to calculate the number of forms to permute the men that are dispposed in the row:
Multiplying all results: