Susan completed 20 courses in college and her grade point average was 3.15 (A = 4.0 and B = 3.0). If all of her grades were A's or B's, and each course grade carried equal weight, what is the number of A's that Susan received?

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We can calculate the weighted average by taking the sum of each value multiplied by its probability. Let's say that Susan got A's in \(x\) percent of her courses.

$$ 4.0(x) + 3.0(1-x) = 3.15 $$ $$ 4x + 3 - 3x = 3.15 $$ $$ x = 0.15 $$

She got A's in 15% of her courses.

$$ 20(0.15) = \boxed{3} $$