Or... you can use the LCM (least common multiple) for your new denominator:

The LCM of 4 and 10 is 20...

Let's make the changes...

So 3/4 is greater than 7/10 again!

How about this one?

Which is greater: 2/5 or 20/49?

Let's use 5 x 49 = 245 as our new denominator (it's the LCM):

89/245 is less than 100/245

So 2/5 is less than 20/49