1.15 miles, or 1 3/20 miles
Two ways to do this: decimals and fractions. I’ll put both in case your teacher wants you to show your work.
Decimals:
3/4 = 0.75 and 2/5 = 0.4
0.75 + 0.4 = 1.15
Fractions:
Need to find the least (smallest number) common denominator to add.
3/4 = 15/20
2/5 = 8/20
15/20 + 8/20 = 23/20 = 1 3/20 miles