Which is bigger 2/3 or 7/8??

When you're comparing fractions, the best thing to do is to either find a common denominator or convert them into fractions. The lowest common denominator between 3 and 8 is 24. 
You've then got to multiply the first fraction by 8, giving you 16/24
You then multiply the second fraction by 3, giving you 21/24
You can also convert them into decimals.
2/3 as a decimal is 0.6666 (recurring)
7/8 as a decimal is 0.875

Therefore, 7/8 is bigger.
Hope this helps :) 

Maybe the answer is 20mm. all i really did was just multiply, which is obvious.