Assume that y varies directly with x, then solve

if y=4 when x=12, find y when x=-24

y= ?

Well how many 12's are in 24 (forget the negative sing for a moment). Two right? Well that means multiply 4 by 2 and get 8, but where does that negative sign go? Just add it! It makes it so much easier taking it away and then putting it back. That is a trick for this problem. y=-8 while x=-24 Hoped I helped!

How do you calculate the mean (average) ?
Isn't it just

  Mean = (total points of all the tests) divided by (number of tests)  ?

Let's work with that.

Multiply each side of that formula by (number of tests), and you have

       Total points of all the tests  =  (Mean) times (number of tests).

In this problem, you know the mean, and you know the number of tests,
so you can easily calculate the total points.