Well, to find the lowest common denominator, you would find the least common multiple.
4, 8, 12, 16, 20
8, 16, 24, 32, 40
16, 32, 48, 64, 80
Now with this, you can see the one they all have in common first is 16.
So, your least common denominator is 16.
Since 7/16 already has 16 as a denominator, you don't change it.
Now, 3/4 would equal 12/16. I got this by changing 4 to 16 and figuring out how many times I multiplied 4 to get 16 (4 times), then I multiplied the numerator by this number.
I am going to do the same thing with 5/8, and I get 10/16.
Hope this helps!