I am getting a #N/A error when using the LOOKUP function in conjunction with
LEFT.

I have a number followed by a letter (e.g., 7442C). I want to compare the
number portion (7442) with a range of numbers (E1:E10), and return the
corresponding description from F1:F10. To drop off the letter portion, I used
LEFT(B5,LEN(B5)-1).

Here's the formula I'm using:
=LOOKUP(LEFT(B5,LEN(B5)-1),E1:E10,F1:F10)
The result is #N/A