not too sure but IMO its D=50%

Let x% answer both I and II correctly, we know 20% answered none. So in remaining 80% we need to find value of x
Those who answered I ONLY = (75-x)%
those who answered II ONLY = (55-x)%
we get, x + (75-x) + (55-x) = 80
therefore, x=50%

i wud also love to know correct answer for this, coz not too sure.

The correct answer is 50%

You can solve this question by using the formula:

Group1 + Group2 + Neither - Both = Total.

In this case, Group1 represents the 75%, Group2 represents the 55%, Neither represents 20%, Both represents X, and Total equals 1, or 100%.

0.75+0.55+0.2-X=1 => 1.5-X=1 => -X=-0.5 => X=0.5 or 50%.

Good luck with the studies!

Adding a little bit more to the previous solution.

For percent problems take 100 as total(very easy to work with)

Using the same formula

grp1+grp2-both+neither = total

75+55-both+20 = 100
both=50

If u draw Venn diagram problems like these would be easy to understand(especially when there is a overlap between 3 sets. This problems had only 2 sets namely first question answer group and second question answer group)

Good luck!