Explanation:
20d + 0.5r >= 1100
in such an equation or inequality we need to relate the same things on both sides. we cannot mix apples and oranges. if we need to bring them together in one expression, we need to transfer them into a common attribute - like weight or (in this case) price.
so, our equality compares actually dollars : the dollars gained by selling tickets, and the total amount of dollars planned to earn ("at least" to create the inequality).
so, $20 per dinner ticket plus $0.50 per raffle ticket must be more than or equal to $1100.
now, for the situation that 50 dinner tickets have been sold (d = 50) we see
20×50 + 0.5r >= 1100
1000 + 0.5r >= 1100
0.5r >= 100
r >= 100/0.5 = 200
r >= 200
so, they need to sell at least 200 raffle tickets to reach the goal.