Question

If 21y5 is a multiple of 9, where y is a digit, what is the value of y? [4 MARKS]

Answer 1

Concept: 1 Mark
Application: 3 Marks

Since 21y5 is a multiple of 9.

Therefore according to the divisibility rule of 9, the sum of all the digits should be a multiple of 9.

$\therefore 2+1+y+5=8+y$

$\Rightarrow 8+y=9\Rightarrow y=1$

This is possible when 8+y is any one of these numbers 0, 9, 18, 27, and so on …….

However, since y is a single digit number, this sum can be 9 only.

Therefore, y = 1