The charge of the tap water supply $W is calculated based on the consumption (c m^3 ) in four months.The first 12m^3 of tap water consumed is free of charge, and the charge for every m^3 consumed afterwards is $4.5.
(a) Write down a formula calculating the value of W for c > 12.
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(b) If John consumed 25m^3 of water in the past four months, how much should he pay ?
**註:上面的>是 greater than or equal to **
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