What is the value of Middle if Left = 12, Right = 24, and Middle is average of both?

Correct — B.
Rule: average of two numbers = their sum ÷ 2.

Sum: 12 + 24 = 36
Average: 36 ÷ 2 = 18 → option (b).

Check: 18 is 6 above 12 and 6 below 24, exactly midway.

Why the others are wrong

• (a)
  15 — 15 is 3 above Left but 9 below Right. The average of two numbers sits the same distance from both.
• (c)
  20 — 20 is 8 above Left and 4 below Right, closer to 24 than to 12, so it is not midway.
• (d)
  22 — 22 is 10 above Left and 2 below Right. It leans almost all the way to Right = 24.

The average (arithmetic mean) of two numbers is their sum divided by 2. On a number line it is the midpoint, equally far from each end.

That gives a second route: take half the gap and add it to the smaller number. The gap is 24 − 12 = 12, half of it is 6, and 12 + 6 = 18.

Key facts

• Average of a and b = (a + b) ÷ 2.
• The average of two numbers is equally far from both: 18 − 12 = 24 − 18 = 6.
• For an evenly spaced list, the average equals the average of the first and last terms.

Common traps

• Stopping at half the gap, 6, instead of adding it to Left: 12 + 6 = 18

No directly related past PYQ was found.