A teacher
teacher's age = t
has two students, one two times as old as the other.
younger student's age = s
older student's age = 2s
The teacher is five times as old as the older student,
t = 5(2s)
t = 10s
and in 5 years
teacher's age in 5 years = t+5
younger student's age in 5 years = s+5
she will be five times as old as her younger student.
t+5 = 5(s+5)
Using t = 10s, substitute 10s for t in
t+5 = 5(s+5)
10s+5 = 5(s+5)
Solve that for s, the age of the younger student.
Then substitute what you get for s in
t = 10s
That will be the teacher’s present age.
Edwin