I'm not sure what you mean but I'll provide both.
5/8 + 3/4
11/8 or 1 3/8.
The common denominator of 5/8 and 3/4 is 8. 3/4 with a denominator of 8 would be 6/8.
5/8 + 6/8 = 11/8 or 1 3/8.
5/8 × 3/4
15/32.
Multiply the numerator by the numerator and the denominator by the denominator.
5 times 3 is 15 and 8 times 4 is 32.
So you get 15/32. This cannot be simplified.