Let x be the missing number 
7/8 = x/48
Cross- multiply 
7/8= x/48
(7)*(48) = x*8
336 = 8x
Flip the equation 
8x = 336
Divide both sides by 8
8x/8 = 336/8
x= 42
Check my answer 
First , replace x by 42
7/8 = 42/48
0.875 = 0.875
I hope that's really help !