dont get this question

6. the probability density function of the net weight in pounds of a packaged chemical herbicide is

f(x)= 2 for 49.75<x<50.25 pounds

b)How much chemical is contained in 90% of all packages?

Please explain?
i got the solutions sheet here but i dont get it!

So: $\ Pr(x \leq X \leq 50.25 ) = 0.90$

OK here is another question :

10. Suppose that the thickness of a cylinder has a normal distribution with a mean 1.41cm and a standard deviation of 0.01cm.

What is the probability that a thickness is greater than 1.42cm?

Can you teach me approximation?

For this, i used the normal distribution equation and i got it wrong . Please try to explain?