A school club sold 220 T-shirts for a fundraiser. Adult-size shirts cost $20 each and child-size shirts cost $15 each. The total collected for all the T-shirts sold was $3,550.

How many adult-size T-shirts were sold?

The answer is "50 adult size t-shirts".

total t-shirts = 220

cost of adult size t-shirt = $20

and cost of child size t-shirt = $15

total money collected when all the T-shirts were sold = $3,550

let x is the number of adult size t-shirts

and y is the number of child size t-shirts, then

x + y = 220

y = 220 - x

and 20x + 15y = $3,550

Now replace y with 220 - x

20x + 15 (220 - x) = 3550

20x + 3300 - 15x = 3550

20x - 15x = 3550 - 3300

5x = 250

x = 50

and y = 220 - x = 220 - 50 = 170

Thus 50 adult size t-shirts were sold.

Assume that the number of adult-size shirts is x and the number of child-size shirts is y.

We are given that:

(1) Total number of T-shirts sold is 220, this means that:

x + y = 220

This can be rewritten as:

x = 220-y ... > equation I

(2) Adult-sized shirts cost $20, child-sized shirts cost $15 and the total collected money was $3550, this means that:

20x + 15y = 3550 ... > equation II

Substitute with equation I in equation II as follows:

20x + 15y = 3550

20 (220-y) + 15y = 3550

4400 - 20y + 15y = 3550

4400 - 3550 = 20y - 15y

850 = 5y

y = 850/5

y = 170

Substitute with y in equation I to get x as follows:

x = 220-y

x = 220-170 = 50

Based on the above calculations,

number of adult-sized T-shirts = x = 50 T-shirts

number of child-sized T-shirts = y = 170 T-shirts