in definition of division it says:

let a and b be two quantities. then the division of a by b is that quantity which, when multiplied by b, produces a.

now my question is "does a and b represents any two numbers?" because if b=0, then how can we find quotient?

If b=0, then 5ab = 0.

Like "Alfred" said: If b = 0, then you can't do "division of a by b" because you can't divide by 0. And you can't do "when multiplied by b, produces a" (unless a = 0 too) because anything times 0 is just 0. So the question doesn't make sense.

Do you mean "if b=1" ?

no alfred,its a different topic

Your question didn't make sense in context, so I think "Alfred" was trying to figure out what you meant. I'm not sure what you're asking either. Do you maybe mean that they should have said that b should be a non-zero quantity, and then the rest follows? So it should be like this:

"Let *a* and *b* be two quantities, **with ***b* non-zero. Then 'the division of *a* by *b*' is that quantity *c* such that, when *c* is multipled by *b*, the result is *a*. In other words, the division is defined as *c*, such that *bc* = *a*."
Like that?