So: If he had biked 56.75 miles, we subtract that from 200 (200-56.75=143.25). Next, we divide 143.25/18.625= 7.691275167785235. 7.691275167785235 rounded would be 8 hours.
I’m not exactly sure what you mean by “write it in math,” however I can tell you what I think. 4y=x (It says that x is 4 times the value of y which means that y times 4 is x.)