Answer

The correct choice:

My sister is 15 months old.

We have to look at the assignment as an equation.

Suppose your sister's age is "X."

Now we can create an equation:

X+5 = 5*X.

Let's solve this equation.

5*X - X = 5

4*X = 5

X = 5/4

Х = 15/12

Since there are 12 months in a year, we can say that the sister is 15 months.

Answer: 15 months.