Hi JJ
The area of that whole triangle is 0.5*10*0.2 = 1
It equals 1! This is very significant. The area under a probability graph MUST equal 1.
P(3 < x < 6)
means what is the probability that the value (x) will lay between 3 and 6.
You can find that by finding the area of the shaded region that is between 3 and 6.
So that would be
P(3 < x < 6) =1 - the region in front where x is less than 3 - the region there x is bigger than 6
P(3 < x < 6) =1 - 0.5*3*0.1 - 0.5*4*0.2
P(3 < x < 6) =1 - 0.15 - 0.4
P(3 < x < 6) = 0.45
OR
you could have done it as a trapezium, like Geno did.
P(3 < x < 6) = 0.5(0.1+0.2)*3
P(3 < x < 6) =0.45
Geno just made a careless error - we all do that sometimes.
I hope that helps. I think these concepts are hard to work out. I remember struggling with them.