In a class of 20 students the average height is 150 cm. If one student of height 155 cm is added to the class, find the new average height.

Correct answer: 150.24 cm. New total = 20 × 150 + 155 = 3155 cm, and new average = 3155 ÷ 21 ≈ 150.24 cm.

Background: The new average = (old total + new value) ÷ (new number of students).

• 151 cm: 151 cm is not 3155 ÷ 21.
• 153.5 cm: 153.5 cm is not 3155 ÷ 21.
• None of these: “None of these” is incorrect because 150.24 cm is correct.

Did you know? Adding a value just above the old average raises the average only slightly, which is why the answer is close to 150.

Memory trick: Total first, then add the new value, then divide.