Katie is twice as old as her sister mara. the sum of their ages is 24. find their ages.

So k=katie's' age
m=mara's age

k is 2 times as old as m
k=2m

sum is 24
add means sum
k+m=24

so we have
k+m=24 and
k=2m
subsitute
2m+m=24
add
3m=24
divide both sides by 3
m=8
subsitute again
k=2m
k=2(8)
k=16

katie=16
mara=8