John has to select a ball from the bag containing 10 balls which are named from 1-10 respectively. If he picks up ball 1, 2 or 3, he will loose 10 points. If he picks up ball 4 or 5, he will get 5 points. But if he picks up ball 6, 7, 8, 9 or 10, he will get 20 points. What is the expected number points he will receive if he randomly takes one ball from that bag?

I guess the answer is:

\left(-10\times \frac{3}{10}\right) + \left(5\times \frac{2}{10}\right) + \left(20\times \frac{5}{10}\right) = -3 +1 +10 = 8 points