To find the answer, you have to convert 7/8 into a decimal or .78 into a fraction so they are in the same form. I'm going to convert .78 into a fraction.
78/100
Simplified it is: 39/50
So, now you cross multiply 7/8 and 39/50 to check it. The number is larger over the 7/8 so you know it is greater.
So, in conclusion 7/8 > 0.78
I hope this helps:)