To find the Least common denominator, find the multiples of each denominator. The smallest number which will appear in each list will be LCD.

Multiples of 4 are 8, 12, 16, 20, 24, 28, 32,...

Multiples of 16 are 32, 48, 64, 80, ,...

Multiples of 8 are 16, 24, 32, 40, 48,....

Here 32 is the smallest number appeared in each list. So LCD of 3/4, 7/16, And 5/8 is 32.

Multiples of 4 are 8, 12, 16, 20, 24, 28, 32,...

Multiples of 16 are 32, 48, 64, 80, ,...

Multiples of 8 are 16, 24, 32, 40, 48,....

Here 32 is the smallest number appeared in each list. So LCD of 3/4, 7/16, And 5/8 is 32.