Question

The original price of a set lunch was 30 dollars. It is now sold at a 20%

Answer 1

20% of $30 = $6
30 - 6 = $24
10% of $24 = $2.40
24 - 2.40 = $21.60 cost of student lunch

You have to do each percent separate to get the cost.
If you just take 30% off from the original cost of $30 it will be the wrong answer.

Answer 2

Find out the total discount percentage that students receive.

20% + 10% = 30%

30/100 x 30 = 9 dollars

30 - 9 = 21 dollars (Final answer)