Question

It was reported that about 80% of airline tickets-nearly $65 billion worth last year are issued over the internet. What were the total sales of airline tickets last year? Round to the nearest billion.

Answer 1

Well we know that 80% of some number, lets call it x, is 65 b.
This means that
x * 80% = 65 b
Divide both sides by 80%
x = 65 b / 80%
= 65 b / 0.8
= 81.25 b

Rounded to 81 Billions