Question

travis is 21 years older than his sister michelle . in 9 years he will be twice as old as michelle how old is each of them now

Answer 1

6 because in 9 years Travis will be 30 and divide that by 2 is 15. 15-9=6

Answer 2

Let T represent Travis's age and M represent Michelle's age

T = M + 21

9 + T = 2M  

use subtitution to replace "T"  with "M + 21" in the second equation.

9 + (M + 21) = 2M

M + 30 = 2M

30 = M

T = M + 21  =  (30) + 21  = 51

Answer: Travis=51, Michelle=30