Answer: They have to sell 21 t-shirts to reach their goal.
T-shirt: $5
Goal: $150
$45 has been raised so far (that means they sold 9 T-shirts) 5 x 9 = $45
150/5 = 30 t-shirts (they need to sell 30 t-shirts in the total)
150-45 = They need $105 to archive their goal
105/5 = 21 t-shirts