The auditorium had 1053 seats. There were 20% fewer $30-seats than $15-seats. 80% of the $15-seats and some $30-seats were sold. A total of $20580 was collected. How many seats were unsold?

	$30	$15	Total
Number	4 u	5 u	9 u

100% - 20% = 80%
⁸⁰₁₀₀ = ⁴₅

Total number of seats
= 4 u + 5 u
= 9 u

9 u = 1053

1 u = 1053 ÷ 9 = 117

Total number of $15-seat tickets
= 5 u
= 5 x 117
= 585

Number of $15-seat tickets sold
= 80% x 585
= ⁸⁰₁₀₀ x 585
= 468

Amount collected from $15-seat tickets sold
= 468 x 15
= $7020

Amount collected from $30-seat tickets sold
= 20580 - 7020
= $13560

Number of $30-seat tickets sold
= 13560 ÷ 30
= 452

Number of tickets sold
= 468 + 452
= 920

Number of tickets left unsold
= 1053 - 920
= 133

Answer(s): 133