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James went to the theatre to watch a concert with his family. 
The theatre had 4270 seats. 
There were 60% fewer $50-seats than $30-seats. 
50% of the $30-seats and some $50-seats were sold. 
A total of $47 500 was collected. How many seats were unsold?
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Let x be the number of the $30-seats in the theater.
Then the number of the $50-seats in the theater is (4270-x).
The problem says that
    4270-x = (1-0.6)x.    ("There were 60% fewer $50-seats than $30-seats")
From this equation, 
    4270-x = 0.4x
    4270 = x + 0.4x
    4270 = 1.4x
    x    = 4270/1.4 = 3050.
Thus, there were 3050 of the $30-seats in the theater and (4270-3050) = 1220 of the $50-seats.
Next, 50% of the $30-seats were sold, which is 3050/2 = 1525 seats,
providing 30*1525 = 45750 dollars of the total revenue.
Hence, the rest part of the revenue, or 47500-45750 = 1750 dollars, was the cost of the $50-seats sold.
So, the number of the $50-seats sold was 1750/50 = 35.
Thus, the number of the total sold seats was 1525 + 35 = 1560.
The number of unsold seats was the rest, or 4270 - 1560 = 2710.
ANSWER.  The number of the unsold seats was 2710.
Solved.