Answer: the 4th boy is 11
Step-by-step explanation:
Let's denote the age of the boys as: x₁, x₂, x₃, x₄.
Then,
Multiply both parts of the equation by denoting by 3:
x₁+x₂+x₃=15(3)
x₁+x₂+x₃=45
Hence,
Multiply both parts of the equation by denoting by 4:
45+x₄=14(4)
45+x₄=56
45+x₄-45=56-45
x₄=11