Showing a function is measurable in a product measure space I'm working on problem 10.G in Bartle's Elements of Integration and Lebesgue Measure:

Let $f$ and $g$ be real-valued functions on $X$ and $Y$, respectively; suppose that $f$ is $\mathcal{X}$-measurable and $g$ is $\mathcal{Y}$-measurable.  If $h$ is defined on $X\times Y$ by $h(x,y) = f(x) g(y)$, show that $h$ is $(\mathcal{X} \times \mathcal{Y})$-measurable.

My problem is that I can't really think of a way to show that any function is $(\mathcal{X} \times \mathcal{Y})$-measurable.  The only thing I can think to do is try to show that
$$
h^{-1}(\alpha,\infty] \in \mathcal{X} \times \mathcal{Y},
$$
but I can't make any progress in this direction.  Any advice would be appreciated.

What you can do is consider functions $\tilde f, \tilde g:X\times Y\to\mathbb R$, where
$$
\tilde f(x,y)=f(x),\ \ \tilde g(x,y)=g(y).
$$
These are easily seen to be measurable:
$$
\tilde f^{-1}(\alpha,\infty)=f^{-1}(\alpha,\infty)\times Y, \ \ \tilde g^{-1}(\alpha,\infty)=X\times g^{-1}(\alpha,\infty).
$$
As $h=\tilde f\,\tilde g$, it is measurable. 
