Answer: Mark up percent = 20%
Step-by-step explanation:
Given: Cost price of each T-shirt = $5.00
Amount required = $450
Total students = 75
Market price for each T-shirt to raise the required amount = (Amount required ) ÷ 75
= $450 ÷ 75
= $ 6
Mark up percent = 

Hence, Mark up percent = 20%
Decrease in the temperature: 5.4×5=27°C
Final temperature: 12-27=-15°C