Mike is three times as old as his little sister, mary. in 4 years, he will be twice as old as mary. how old is mary?

Mary: x yrs old
Mike: 3x yrs old

In four yrs:

Mary: x+4 yrs old
Mike: 3x+4 yrs old

As the text of task:

3x+4=2(x+4)
3x+4=2x+8
3x-2x=8-4
x=4

Mary is 4 yrs old

X-the age of Mike
y-the age of his sister

x=3y

x+4=2(y+4)
3y+4=2y+8
3y-2y=8-4
y=4 years

x=3*4=12 years

C. 40,320 just did the math right now