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If the total amount of money is 20.25$, but it has to be counted in only dimes and nickels, 
how many dimes and nickels are there if there are four times the amount of dimes than there are nickels?
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Combine coins (mentally) in groups by 4 dimes and 1 nickels in group.
Each group worth is 4*10+5 = 45c.
Divide 2025 by 45:  = 45.
Hence, you have 45 such groups.
It means that there are 45 nickels and 4*45 = 180 dimes.