Hi Aryanna, I believe 3:4 is a typo. I assume you mean either 3/4 inches or 3.4 inches. I will solve this as if you meant 3/4 inches. If it was supposed to be 3.4, then replace my 3/4 with 3.4.
For this type of problem, you can use proportions.
1in/5miles = (3/4in)/x
Now, cross multiply.
1in * x = (3/4)in * 5miles
Now multiply our the fractions on your left hand side.
x in =(15in*miles)/4
Now divide both sides by inches to isolate x.
x = 15miles/4
Therefore, x = 15/4 miles = 3.75miles