Select the greatest number below a given number using Python - only using while loops, if/elif, for loops

I am a newbie to Python and trying to find the greatest number below a given number. Under normal circumstances I would use lists like in the link below: Select the greatest number below a given number using LINQ to Objects?

However, we are only allowed to use while and for loops, if/elif states, and functions. We cannot use lists.

Basically, the problem is as follows:

1. Prompt user to enter a positive integer x.
2. Find the highest value v that is less than or equal to the number x and its corresponding alphabet letter n:

Value (v) Letter (n) 1 a 4 b 5 c 9 d 10 e 40 f 50 g 90 h 100 i

This is the first part and it has me stuck. I was thinking of using the partial code below but I am not sure if it is going in the right direction.

assume x = 29

y= 1
while y <= x
    if y <= x
         y = 4
    elif y = 1

The problem I am having is how to do continue checking the entire list of numbers (1,4,5,9,10, etc.) Any suggestions is appreciated. Thanks.