Shortcut Trick
The extra amount spent by the 20th student over the average is shared by the other 19 students.
Increase in average per student = Extra amount ÷ Remaining students
⇒ Increase = 19 ÷ 19 = ₹1
Actual Average = Individual spend + Increase = 175 + 1 = ₹176
Total money spent = Total students × Average = 20 × 176 = ₹3,520
∴ The correct answer is ₹3,520.
Alternate Method
Given:
Total number of students = 20
Amount spent by 19 students = ₹175 each
Amount spent by 20th student = Average + ₹19
Formula Used:
Total Expenditure = Average (A) × Total Students (n)
Let the average expenditure of all 20 students be x.
⇒ Total expenditure = 20 × x
⇒ Total expenditure = (Sum of 19 students) + (Expenditure of 20th student)
⇒ 20x = (19 × 175) + (x + 19)
⇒ 20x = 3325 + x + 19
⇒ 20x − x = 3344
⇒ 19x = 3344
⇒ x = 3344 ÷ 19 = 176
⇒ Total money spent = 20 × 176 = 3520
∴ The correct answer is ₹3,520.
Additional Information
Concept of Average
Average is the sum of all observations divided by the total number of observations. Average = Sum ÷ n.
Net Deviation Method
The sum of deviations of all observations from the Arithmetic Mean is always zero. This property is used to solve complex average problems quickly.
Weighted Average
When different groups have different averages, the combined average is (n1A1 + n2A2) ÷ (n1 + n2).