The entire race is 1 full race.
One full race is 1.
He ran 3/8 of 1, so he ran 3/8.
He needs to run the rest of the race.
The rest is unknown, so we call it x.
When you add 3/8 to the unknown, you get the full race.
The equation is
x + 3/8 = 1
Change 1 to a denominator of 8.
x + 3/8 = 8/8
Subtract 3/8 from both sides.
x = 5/8
Answer: He still needs to run 5/8 of the race.
