2feet
Step-by-step explanation:
To the greatest length, in feet, that he can make them, we need to find the greatest commom factor betwwen 4 and 18
Pairs of factor of number 4 : 4 and 1, 2 and 2 Pairs of factor of number 18: 1 and 18, 2 and 9, 3 and 6So only 2 are the greatest commom factor, then the greatest length is 2feet with no yarn left over!