Here's another one I was asked about. These aren't that hard, they just require some reasoning. A tip, substitute in some numbers that work to help you see the relationships. Define your variables to help you keep them straight.
x= # boys
y = # girls 2x = y
You won't be solving, but helps you see relationship. So think, if there are 10 girls in the class, that means there are 20 boys. So, 1/3 of class is girls and 2/3 is boys.
Now you can move on - All of the girls are in math club so that is 1/3 or 33 1/3%. Half of the boys, so half of 2/3 is 1/3 or 33 1/3%
Add 33 1/3% + 33 1/3% and get 66 2/3%, D