Glossarytaught in M0.03, CN2.01
Back of the envelope
Also called back-of-the-envelope, rough calculation.
A quick calculation from a few remembered quantities, done to establish the size of an answer before any detail.
Start with 140 crore people, take a 30% addressable share and give each ₹2,000 of annual spend. That sizes an Indian consumer category at about ₹84,000 crore, which is enough to know whether a company's claim is plausible.
Skill shows in choosing quantities you actually know and being explicit about each step, so a reader who doubts the estimate can correct it.
Speed matters less than being able to show the chain.