The missing justification is for the statement that three angles add to a particular angle. The appropriate choice is ...
... c. Angle Addition Postulate
"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.
45
15 is a multiple of 3
30 is a multiple of 3
45 is NOT a multiple of 3
60 is a multiple of 3
Ect.
Hey! I'll provide you the answers!
First you want to simplify the equation using cross-multiplication.

Finally, you would want to divide both sides by 15.
n = 161
It would be 12 ones it would be this because you have to add 7+ 5