If 21y5 is a multiple of 9, where y is a digit, what is the value of y?

If 21 y5 is a multiple of 9, where y is a digit what is the value of y ?

If 21y5 is a multiple of 9, where y is digit, what is the value of y?

If 21y5 is a multiple of 3 ,where,y is a digit,what is the value of y.

If 21y5 is a multiple of 9,when y is a digit,what is the value of y?

If bar2y5 is a multiple of 11,where y is a digit,what is the value of y?

If 21z5 is a multiple of 9,where z is a digit,what is the value of z.

If 31z5 is a multiple of 9, where z is a digit, what is the value of z?

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?