Mr. harris is building an addition to his house. to start, he needs 300 eight foot long 2x4's and 60 ten foot long 2x4's. how many yards of 2x4's will mr. harris purchase?

We know that

Mr Harris needs
300 eight foot long 2x4's> 300*8=2400 ft
60 ten foot long 2x4's > 60*10=600 ft
[2400+600]=300 ft

remember that 
1 yard > 3 foot
 X> 300 ft
X=300/3=100 yard

the answer is 
Mr. Harris will purchase 100 yard of 2x4's