I agree with Dookie - it's 80%. I have a non-algebra approach that works for this exact problem.

Draw a quick diagram putting all the percentages together that looks like this:

40------42-----------50

W_____t_________M

Then see what the differences are between all the numbers and the middle:

40------42-----------50

W__2__t_____8___M

Now make a ratio using the opposite differences:

w:m = 8:2

so w:whole = 8:10 = 80%

It looks like a lot when it's typed out like this, but see the attachment for how simple it is:

This works for every question where they give you the percentage of two distinct groups, and then the combined percentage as well.