In a classroom, the average height of all the boys was 170 cm. Rahim recently…
2025
In a classroom, the average height of all the boys was 170 cm. Rahim recently joined the class, which originally had 20 boys, and increased the average height to 171 cm. Find the height of Rahim.
- A.
171
- B.
181
- C.
191
- D.
201
Show answer & explanation
Correct answer: C
Concept: When a new member joins a group and the average changes, the new member's value equals the difference between the new total and the old total: New total = new average x new count of members; Old total = old average x old count of members; New member's value = New total - Old total.
Application:
Old total height of the 20 boys = 170 x 20 = 3400 cm.
After Rahim joins, the class has 21 students and the new average height is 171 cm, so the new total height = 171 x 21 = 3591 cm.
Rahim's height = New total - Old total = 3591 - 3400 = 191 cm.
Check: (3400 + 191) / 21 = 3591 / 21 = 171 cm, which matches the given new average, confirming the value found above.