Question: Problem:

* I have 10 cells (A1-A10) in my workbook where I can enter values. The values are not sorted (always random).
* I have 1 cell (C4) where the average of that value is calculated with the formula: `=AVERAGEIF(A1:10,"<>0")`
* I need a formula for the cell C5 where this average is rounded to a **lower** step value (unless its already equal to one of the step values).
* My step values are:
1,0
1,3
1,7
2,0
2,3
2,7
3,0
3,3
3,7
4,0

Examples:

* If my average value (C4) is 3,2 I need it to show in the field C5 as 3,0.
* If my average value (C4) is 1,3 it needs to stay 1,3

Answer: 
While you can have multiple NAT instances inside a single AZ, each routing table (associated with a subnet) can only have a single entry for the route `0.0.0.0/0`.

The HA approach is normally to have 2 NAT instances (one active, the other one on standby), which check each other. If the standby instance cannot reach the active one, it will replace the other instance's ENI on the `0.0.0.0/0` routing entry.
I met a related question in Solution Architect exam tests. You can surely have as many NAT instances in 1 AZ as you want, but anything that is replicated inside 1 AZ is not considered HA.

Since NAT is related to Egress control here are a couple of articles about that:

* [Controlling Egress for VPC](https://aws.amazon.com/answers/networking/controlling-vpc-egress-traffic/)
* [How to add dns filtering to your nat instance with squid](https://aws.amazon.com/blogs/security/how-to-add-dns-filtering-to-your-nat-instance-with-squid/)
* [Enforcing a squid access policy for amazon s3](https://aws.amazon.com/articles/enforcing-a-squid-access-policy-for-amazon-s3-and-yum/)

As pointed out in Xeraa's answer there's no way to have multiple NAT instances simultaneously because the whole idea of having NAT is to ensure that **all traffic** goes through it. And at the same time there is a limitation that there can only be one default route. So for HA solution you either need:

* Extra LB
* Instance-level configuration per each instance
* or a managed
NAT Gateway

Even with an extra ELB it's gonna require some tweaks per instance since you won't be able to point your RTs to ELB. Here is what you can use as a destination for Route Table:
[![enter image description here](https://i.stack.imgur.com/UsO67.png)](https://i.stack.imgur.com/UsO67.png)