Question

Will is twice as old as Jill. Three years ago, Jill's age was two fifths of Will's age then. How old is each now?

Answer 1

"Will is twice as old as Jill."

                       Jill's age . . . . .   J
                       Will's age . . . . 2J .

"Three years ago . . .

                       Jill's age then . . . . .   J - 3
                       Will's age then . . . . 2J - 3

". . . Jill's age then was 2/5 of Will's age then."

                                         J - 3  =  (2/5) (2J - 3)
Multiply
each side by  5 :          5J - 15  =  2 (2J - 3)

Divide
each side by  2 :          2.5 J - 7.5  =  2J - 3 

Subtract  2J
from each side:            0.5 J - 7.5  =      -3

Add  7.5
to each side:                0.5 J           =     4.5

Multiply
each side by  2 :                J            =       9

Jill is  9  y.o. now.
Will is 18 y.o. now.