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    A public telephone has been vandalised and will take only 50 pence coins and 10 pence coins. A
    woman puts £8.50 into the phone to make a long-distance call with twenty-five coins. Five 10
    pence coins are returned when she finishes her call.
    What combination of coins is retained by the machine?
    A Twice as many 10s as 50s.
    B One-and-a-half times as many 50s as 10s.
    C The same number of 50s and 10s.
    D Three times as many 50s as 10s.
    E One-and-a-half times as many 10s as 50s.

    The answer is D but why?? Thank you for your help!
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    Let x be the number of 50p and y the number of 10p coins initially inserted.
    Then we can set up a simultaneous equation:
    0.5x + 0.1y = 8.5, and x+y = 25 (as 25 coins are inserted)
    Solving this quickly, x=15 and y=10.
    Then we know that 5 10p are returned, so the machine has retained x 50p coins (15) and y-5 10p coins (5) which is the 3:1 ratio that option D provides
 
 
 
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