(a) Proportion of students that got a B or higher.
Add the percentages or decimals for X=3, 3.3, 3.7, 4: 16+6+7+19=48% or 0.48.
(b) Grade poorer than B- is when X<2.7, P(X<2.7).
Add the percentages or decimals for X=0.0, 0.7, 1.0, ... 2.3:
3+4+5+3+5+15+5=40% or 0.40.
The 4-point scale is just a convenient way of converting the grade letters into numbers. This means that results are easier to computerise, because computers are better working with numbers than letters; whereas most humans find the grade letters have more meaning to them. The other thing students may find confusing is the function P, but it's only shorthand for using the words "the probability of" then parenthesising the particulars, X is equal to, greater than, less than, etc., where X is the numerical value associated with the grade. This question is testing your understanding of how to convert ordinary words into mathematical terms, and vice versa. So don't let this sort of question put you off. It's really simpler than you think!
For example, P(X<2.7)=0.40 is so much shorter than using a lot of words, and yet it means the same thing. Read More: ...