5 Equations in 5 Days: Cost of Gmail

This is part of a mini-series illustrating how to think about estimation prompts for Google interviews. It is also a way to build your logic instincts.

Instructions:

1. See prompt below

2. Think about how you would estimate the answer

3. Identify the variables in your thoughts

4. Derive a simple equation

What you should avoid:

1. Overthinking

2. Trying to get it perfect

Now let’s dive in:

What is the problem:

Estimate how much Gmail users cost Google?

What are the key elements:

• costs

  • infra

  • development

• # users

What are the variables?

• X = infra costs (variable)

• Y = development costs (fixed/not variable by user)

• Z = # users

What’s the equation

=(X*Z )+ Y

Note: Yes, more developers are put on the product as it grows and there are some development costs that change as users grow but developer costs start to get into the world of zero marginal cost assumptions.

Another POV on this Prompts

Here is Stellar Peers’ answer to a similar version of the prompt. Why I like this answer: they show you two different approaches.

Further Reading

Today we only care about coming up with an equation. The point of these very simple equations is to help you develop a baseline approach to the problem before worrying about the more complex assumptions. If you want more on how to come up with the numbers to put in your equation, read more here.

BONUS:

It is a good idea to have a cheatsheet at your fingertips for estimation questions. Here is an article with links to cheat sheets for quick facts.