Answer: the 4th boy is 11
Step-by-step explanation:
Let's denote the age of the boys as: x₁, x₂, x₃, x₄.
Then,
[tex]\displaystyle\\\frac{x_1+x_2+x_3}{3}=15[/tex]
Multiply both parts of the equation by denoting by 3:
x₁+x₂+x₃=15(3)
x₁+x₂+x₃=45
Hence,
[tex]\displaystyle\\\frac{x_1+x_2+x_3+x_4}{4} =14\\\\\frac{(x_1+x_2+x_3)+x_4}{4} =14\\\\\frac{45+x_4}{4}=14[/tex]
Multiply both parts of the equation by denoting by 4:
45+x₄=14(4)
45+x₄=56
45+x₄-45=56-45
x₄=11
