Calculate tiered discount amount using a CodeIgniter query

I have a discount lookup table like this:

Basically the tiered discount lookup works like this:

• The 0 to 3 days discount is 5.00
• The 4 to 7 days discount is 10.00
• The 8 to 10 days discount is 15.00

If the passed-in days number is 5, then the query should return 10.00.

How can I write a CodeIgniter query to perform this logic?

SELECT discount FROM mytable WHERE days > XXXX LIMIT 1 where XXX is the number of days you want.