Answer:
31 girls
Explanation:
The way this question is worded, one can see that one will likely be able to calculate the number who bought pens of all three colors, the number who bought pens of only two colors and the number who bought pens of only one color.
The total number of girls will equal the following.
The Number Who Bought Only One Color + The Number Who Bought Only Two Colors + The Number Who Bought All Three Colors
Let's break down what the question says to get some subtotals.
Three bought one of each colour.
-- 3 Bought All Three
Of the girls who bought two colours, 3 did not buy red, 5 not green and 2 not black.
-- 3 Bought Only Black And Green
-- 5 Bought Only Black And Red
-- 2 Who Bought Only Red And Green
The same number of girls bought red only as bought red with other colours.
I guess this means the following.
-- 3 Bought All Three
-- 5 Bought Red And Black
-- 2 Bought Red And Green
-- 3 + 5 + 2 = 10 Bought Only Red
More girls bought red and black but not green than bought black only,
-- < 5 Bought Only Black
more girls bought green only than bought green and black but not red.
-- > 3 Bought Only Green
The same number bought black only as bought green only.
-- 3 < Only Green = Only Black < 5
-- 4 Bought Only Green
-- 4 Bought Only Black
Total:
3 All Three
3 Only Black And Green
5 Only Black And Red
2 Only Red And Green
10 Only Red
4 Only Green
4 Only Black
_____________________
31 Girls
