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
