Instead you ask her to multiply her age by 10, then subtract any of the first nine multiples of 9 (9,18,...,81), and tell you the result. Based on this number N, find the lady's age.
Example: input N=384, output a=42. Indeed, 42*10 - 36 = 384.
The function is not injective, the same output age a corresponds to multiple input values N.