(4/7) / (10/21):
Dividing 2 fractions is equal to multiplying the first fraction by the inverse of the second: Inverse of fraction a/b is b/a, so the inverse of 10/21 is 21/10.
So (4/7) / (10/21) = (4/7) * (21/10)
Now you multiply the numerators between each others to get one numerator, and same thing for the denominators
(4/7) * (21/10) = (4*21) / (7*10) = 84/70.
Now we need to simplify the fraction we got. Both 84 and 70 can be divided by 14, so 84/70 = (84/14) / (70/14) = 6/5.
So the (4/7) / (10/21) = 6/5
Hope this Helps! :)