If 31z5 is a multiple of 9, where z is a digit, what is the value of z? You will find that there are two answers for the last problem. Why is this so?

Since is a multiple of 9. 315 z
Therefore, according to the divisibility rule of 9, the sum of all the digits should be a multiple of 9.
∴ 3+1+z+5=9+z ⇒ 9+z=9 ⇒z=0
if 3+1+z+5=9+z ⇒9+z=18 ⇒z=9

Class 8 Maths Chapter 11 Exercise 16.2 Solution in Video

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https://www.tiwariacademy.com/ncert-solutions/class-8/maths/chapter-16/