How do you do this 4/9 X 900/1 ?

Multiply the numerators then the denominators. So, $${\mathtt{4}}{\mathtt{\,\times\,}}{\mathtt{900}}$$  and  $${\mathtt{9}}{\mathtt{\,\times\,}}{\mathtt{1}}$$ .  That makes  $${\frac{{\mathtt{3\,600}}}{{\mathtt{9}}}}$$  . Simplify that into  $${\frac{{\mathtt{400}}}{{\mathtt{1}}}}$$ , which is 400. That's you're answer!

That's cross multiplication by the way, tried to post earlier but didn't come through apparently sorry!