A city water board charges nothing for the first 10 kilolitres used in a month, ₹8 per kilolitre for the next 15 kilolitres, and ₹15 per kilolitre for everything beyond 25 kilolitres. A household used 40 kilolitres last month. What is its bill, in ₹?

Split the 40 kilolitres across the three slabs: the first 10 are free, the next 15 cost $15\times 8$ = ₹120, and the remaining 40 − 25 = 15 cost $15\times 15$ = ₹225. The bill is 0 + 120 + 225 = ₹345. Charging the first 25 kilolitres at ₹8 and forgetting the free slab gives 200 + 225 = ₹425; putting everything above 10 kilolitres into the top slab gives $30\times 15$ = ₹450; and ₹225 is the top slab on its own.