On Saturday, Craig rode his bike 5/8 of a mile. On Saturday, he rode his bike 1/2 of a mile. Craig added 5/8+1/2=6/10and plotted 6/10 on his number line.

* Craig's answer is not reasonable because to add fractions the denominators must be the same.

** Total distance = 5/8 + 1/2 = 5/8 + 4/8 = 9/8 miles

*** Using the line number to prove the answer:
The line number that represents the problem is in the attached figure.
while the distance between 0 and 1 divided to 8 sections
to represent (5/8) count 5 sections from zero ⇒⇒⇒ point (a)
and to represent (1/2) it is the midpoint between 0 and 1 which mean it is 4 sections but it will be counted from point (a) so, adding 4 sections to point (a) the result will be the point (b)
So, counting from 0 to point (b) will give us 9 sections
and while one section represents (1/8)
So the total distance will be 9 * (1/8) = 9/8 which is agree with the result obtained before