Question

if 21y4 is divisible by 3 then .What is the value of y

Answer 1

A no. is divisible by 3 only when it adds up to 3 or a factor of 3 so 
2+1+y+4 can be 2+1+2+4= 9 
So the value of y can be 2
2+1+5+4= 12
So the value of y can be 5 too
Similarly y can have many values i.e. 2,5,8 etc.

Answer 2

the sol. is
=> 21 × y × 4 is divisible by 3
=> 84 × y divisible by 3
=> so to find the value of y we will see each no. by multiplying it to 84.
=> 84 × 2= 168 which is not divisible by 3 so then we will move on to other no.
=> 84 × 3 = 252 which is divisible by 3 so the value of y will be 3