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)∗3T+y=3S+3y|−3ST+y−3S=3y|−yT−3S=2y|:2y=T−3S2y=35−3∗52y=35−152y=202y=10
In 10 years: Mr Tanaka is 45 and his son is 15.
15 * 3 = 45