The bicylist rode 4.5 miles in 18 minutes.
To convert 4.5 miles in 18 minutes we can divide 60( 1 hour) by 18 which is 10/3 , and multiply it by 4.5 miles:
4.5(10/3) miles in 18(10/3) minutes = 15 miles/h
Which means that he rode at a speed of 15 miles/h for 18 minutes
Then he rode 7.5 miles in 30 minutes which means he rode at an avarage speed of 15 miles/h for half an hour.
7.5(2) miles in 30(2) minutes
Now to find the avarage of
15 miles/h for 18 minutes
15 miles/h for half an hour
As both speeds are the same the average is 15 miles an hour
I hope you understood my brief explanation