They knew that they could 3 miles in 20 minutes. how far can they ride in 45 minutes?

45 minutes and 20 minutes have to compared. 
45/20= 2,25 
That tells me that if I multiply 20 minutes by 2,25 I get 45 minutes. 
Thus if you travel 3 miles in 20 minutes you will travel 3*2,25 miles in 45 minutes. 3*2,25=6,75