School trip

On a school trip, 17 of the 28 children bought ice cream or chocolate in a candy store. Twelve children bought chocolate, and nine children bought ice cream. How many children bought ice cream and chocolate? How many children did not buy ice cream? How many children did not buy chocolate?

Result

a =  4
b =  19
c =  16

Solution:

$a = 12+9-17 = 4$

$n = 28-17 = 11 \ \\ b = n + (12-a) = 11 + (12-4) = 19$

$c = n + (9-a) = 11 + (9-4) = 16$

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