How many transactions per second does a Mint-scale finance platform process?

The scenario

You’re designing the transaction ingestion pipeline for a Mint-like personal finance platform. The service aggregates financial data for 10 million users, each with an average of 3 linked bank/credit accounts.

Across all users and accounts, the platform processes 5 billion financial transactions per month — with a 10:1 write-to-read ratio (people spend daily, but rarely open the app to review).

What is the average sustained transaction write throughput in transactions per second?

Adapted from: donnemartin/system-design-primer — Design Mint.com — CC BY 4.0