Mr. Tanaka is 35 years old, and his son is 5. In how many years will Mr. Tanaka be three times as old as his son?

Mr. Tanaka = T

Son = S

years = y

$$\\T+y=(S+y)*3\\
T+y= 3S+3y \quad | \quad -3S\\
T+y-3S=3y \quad | \quad -y\\
T-3S=2y \quad | \quad :2 \\
y=\dfrac{T-3S}{2}\\
y=\dfrac{35-3*5}{2}\\
y=\dfrac{35-15}{2}\\
y=\dfrac{20}{2}\\
\boxed{y=10}$$

In 10 years: Mr Tanaka is 45 and his son is 15.

15 * 3 = 45