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Explanation:
Let's solve the given equation for the variable 'a'
a(2+1) = 36
a*(3) = 36
3a = 36
a = 36/3
a = 12
There are 12 students in the class. This must mean there are 12 lemonades, because each person gets 1 lemonade.
Since there are 24 cookies, each student gets 24/12 = 2 cookies
Since each student gets 2 cookies and 1 lemonade, this is where the "2+1" comes from in the original equation. Each student gets 3 items total, which explains the notation 3a.