10 people went to a restaurant to have a meal as a group. Of these, 9 had already contributed Rs 400 each, while the 10th person had to give the entire additional amount required to pay the restaurant's bill. How much did this person pay if he had to pay Rs 900 more than the group's average?

Averages — Preparation

This question tests your understanding of Averages, a key topic in the Quantitative Aptitude section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.

An average is the sum of a set of values divided by the number of values. Average questions in entrance exams go well beyond this basic formula: they test what happens when a person joins or leaves a group, when a wrong value is corrected, the average age of a family over time, weighted averages of two groups, and averages of consecutive numbers. The fastest approach is the deviation method. Instead of adding every value, assume an average, note how far each value is above or below it, and adjust. For example, if a new member raises a group's average by 2, the new member is 2 times the new group size above the old average. Questions on cricket scores and batting averages are a common variation. Practise the average questions below, each with a full solution and a shortcut using the deviation method where it applies.