Pierre ran 19 and 13/20 miles.
Given:
marathon length : 26 2/10 miles
pierre's length ran : 3/4 of the way.
Multiply the fractions.
Convert the marathon length from a mixed fraction into a fraction.
26 2/10 = ((26*10)+2)/10 = (260+2)/10 = 262/10
262/10 * 3/4 = (262*3) / (10*4) = 786/40 = 19 26/40
19 26/40 simplified to 19 13/20 miles
26 ÷ 2 = 13
40 ÷ 2 = 20