यदि किसी संख्या में 5% की वृद्धि करने पर वह संख्या 21 हो जाती है, तो मूल संख्या क्या है?

A person saves 90% of his income. If his expenditure is ₹520, find his income (in ₹).

By how much is 30% of 60 greater than 25% of 40?

By how much is 30% of 50 greater than 35% of 30?