The common denominator for both fractions is 100. You have to multiply the numerator and denominator of the first fraction (21/25) by 4, and the numerator and denominator of the second fraction (17/20) by 5 to obtain this denominator.
After doing that, you should have the fractions 84/100 and 85/100. The latter is the larger one, so your expression would be:
21/25 < 17/20
Hope this helps! If you need some more help understanding this, please reach out to me.
