AWS Certified Solutions Architect Associate Test 3

Since multi-AZ RDS needs 2 private subnets to provide high availability and 2 public subnets are needed for ELB(web-tier) application.
Would use VPC with private (DB) and public (WEB) subnets:
http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_VPC.Scenarios.htmlMultiAZ requirement forces me to multiply subnets by two Reasons:

For DB: Your VPC must have at least one subnet in at least two of the Availability Zones in the region where you want to deploy your DB instance. A subnet is a segment of a VPC’sIP address range that you can specify and that lets you group instances based on your security and operational needshttp://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_VPC.WorkingWithRDSInstanceinaVPC.html

For Web: After creating a VPC, you can add one or more subnets in each Availability Zone. Each subnet must reside entirely within one Availability Zone and cannot span zoneshttp://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.html