Question 527199
With coin problems, you have to keep track of the count of coins and their values.
d = number of dimes
10d = value of dimes in cents
q = number of quarters
25q = value of quarters in cents
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10d + 25q = 1340 cents
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Mike has 3 time as many quarters as dimes, so we would state
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q = 3d
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10d +25(3d) = 1340
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10d + 75d = 1340
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85d = 1340
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d = 1340/85
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At this point, the problem is that 1340/85 is not an integer.
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Since you cannot have fractional dimes, you have to successively determine the point where you get an integer from the division.  You can do this by subtracting 100 cents (1 dollar) again and again.  Or you might notice that 85*4 = 170*2 = 340.  
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That would mean Mike has 10 $1 bills in his pocket, 4 dimes and 12 quarters.
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Check the solution to determine if it is the answer.
25(12) = 300 cents
10(4) = 40 cents
10 * 100 = 1000 cents
300 + 40 + 1000 = 1340 cents = $13.40
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Done.