He will need to buy 3/4 lb. more ham.
First we find out how many cheese sandwiches he can make. He uses 1/6 of a pound of cheese on each sandwich:
5÷1/6 = 5/1÷1/6 = 5/1×6/1 = 30
He can make 30 cheese sandwiches.
Let x be the extra ham he needs to buy:
(3+x)÷(1/8) = 30
Multiply both sides by 1/8:
3+x = 30/1×1/8
3+x = 30/8 = 3 6/8 = 3 3/4
Subtract 3 from each side:
x= 3 3/4 - 3 = 3/4