A shop's monthly power bill came down from ₹1,250 to ₹1,000. The saving is what per cent of the amount now payable?

The saving is ₹250, and the words 'of the amount now payable' fix ₹1,000 as the base: 250/1000 = 0.25 = 25%. 20% is 250/1250, the same saving measured against the OLD bill - that is the fall in the bill, a different question. 80% is the new bill as a share of the old one, and 125% is the old bill as a share of the new. One gap, four ways of quoting it: always read which figure the question puts after the word 'of'.