Suppose we have a bag with 10 slips of paper in it. Eight of these have a 2 on them and the other two have a 4 on them. How many 4's do we have to add to make the expected value equal to 3?

2 * (8/10) + (4 + x) (2/10) = 3

Where x is the number of additional slips showing "4"

Simplify the above.....and we get

8/5 + (4 + x) (1/5) = 3

8/5 + 4/5 + x /5 = 3

12 / 5 + x / 5 = 3 [ multiply through by 5 ]

12 + x = 15

And it's obvious that x = 3