85%
Justin has ridden 34 miles out of 40 miles. This can be expressed as a fraction, 34/40.
If you carry out the division 34 ÷ 40, you get 0.85. This is the proportion of the course that Justin has ridden, and is always less than one. To convert this to a percentage, you multiply it by 100:
0.85 * 100 = 85%