Check out my eHow article on how to add fractions with different denominators and feel free to click on any ads/links of interest:
http://www.ehow.com/how_4722323_two-simple-fractions-different-denominators.html
You need a common denominator. What number is a multiple of 2, 4, AND 5? It's 20 since:
multiples of 2=2,4,6,8,10,12,14,16,18,20,...
multiples of 4=4,8,12,16,20,...
multiples of 5=5,10,15,20,...
20 is a common denominator since it appears in ALL THREE lists. Now, 2/5=8/20, 1/4=5/20, and 1/2=10/20. The problem is:
(8/20)+(5/20)=x-(10/20)
13/20=x-(10/20)
Add 10/20 to both sides:
x=23/20 or 1 3/20