Can someone tell me how to do the following inside a unix script?

I have a file and can do a wc -l on it....say the count is 123 lines.

#wcl /tmp/file
>123

I would then like to divide this number by 2 to get a decimal number.

# wcl /tmp/file| `expr /tmp/file /2`

Can't find the right expr combo.

Any idea how to accomplish this?

Thx.
Kurt