The lead of a screw is the distance that the screw advances in a straight line when the screw is turned 1 complete turn. if the screw is 2 1/2 inches long and has a lead of 1/8 inch, how many complete turns would get it all the way into a piece of wood?

Based on the given problem the lead is 1/8 inch
 so with one turn the screw advances 1/8 of an inch

to find the number of turns needed you would need to divide the length ( 2 1/2) by the lead (1/8)
 first step is to turn  2 1/2 into an improper fraction by multiplying the whole number by the denominator and adding the numerator and using that number as the new numerator:

2 1/2 = =2*2 = 4 + 1 = 5/2

 now divide 5/2 x 1/8
 to divide 2 fractions, flip the second one over and then multiply straight across:

5/2 / 1/8 becomes 5/2 x 8/1

8x5 = 40 and 2x1 = 2 so you would have 40/2 which reduces to 20

this means it would take 20 complete turns