This is my very first post. I am a bit nervous because I am unsure if
I am creating this post correctly. Anyways, I need help on a formula
that will do the following:

Lets say that there are 100 Total Lunch days. As part of the 100 Total
Lunch days, there are 10 Sandwich days.

The Sandwich day rate is currently 10%.

The new target Sandwich day rate is 4.3%.

To match the new target Sandwich day rate, I need to figure out the NEW
Total Lunch days and the NEW Sandwich days.
Sandwich Days/Total Lunch days would be 4.3%.

What formula would I use to do this?
I am using excel.

Thanks.