JAMB - Mathematics (1994 - No. 2)
The mean of twelve positive numbers is 3. When another number is added, the mean becomes 5. Find the thirteenth number
29
26
25
24
Explication
Let the sum of the 12 numbers be x and the 13th number be y.
\(\frac{x}{12} = 3 \implies x = 36\)
\(\frac{36 + y}{13} = 5 \implies 36 + y = 65\)
\(y = 65 - 36 = 29\)