/sci/ - Science & Math

>question
If Mary is 16 years old, which is 4 times as old as her brother, how old is Mary?
>answer
16 / 4 = 4 so Mary is 16; her brother is 4.
Mary will be twice as old as her brother when the age of her brother is equal to the number of years between their ages.
16 – 4 = 12 so Mary will be twice as old as her brother when her brother is 12.
12 + 12 = 24 so Mary will be 24.


can someone explain to me why "Mary will be twice as old as her brother when the age of her brother is equal to the number of years between their ages." is true? i see that it works out but how did he come up with that rule?