Not sure if my interpretation is correct but here goes :

①'Earned' refers to profit.
② 'Gave away' refers to those free t-shirts given to people who bought 2.

Since 8 t-shirts were given away,

8 ÷ 1 = 1

8 customers bought 2 t-shirts each.

Profit from 1 such customer

= $20 × 2 (amount collected from the 2 shirts, thirds one is free) - $9 × 3 (cost price of 3 t-shirts)

= $40 - $27
= $13

From 8 such customers,

8 × $13 = $104

Profit from the other customers who only bought 1 t-shirt
= $335 - $104
= $231


For these customers, profit per customer
= $20 - $9 = $11

Number of such customers

$231 ÷ $11 = 21

So, 21 customers bought only 1 t-shirt.