What if s=0?
s can't be zero in this question because n/s =4, which is defined.
if s =0, n/s would be infinity, which is not the case here.
The same to me, we are falling in the trap. I wonder why it is easy, but it still be posted here? And I found the trap very interesting. Worthy to know it!
When we assume n/s =4 that n = 4s, the condition attached to it must be s is not zero. But nobody tell us that s is none zero. So it is not sufficient to solve!