Question

When do we use 3.14 as the value of pie instead of 22/7

Answer 1

If you see in your problem that there are a lot of fractions, then you can go ahead and use 227 so that you could cancel some terms out and simplify your calculations.

But if you see in your problem that there are decimals and whole numbers you can go ahead and use 3.14.

But again, it's just a matter of preference. You could also write,
3.14=314100.

The goal mainly is to simplify your calculations and make it easier for you to carry on the arithmetic or algebraic operations. The only logic is to see which will benefit you the most and save your time while solving a particular problem.

Answer 2

It is mostly recommended to use 22/7 but when the question already mentions to use 3.14 then you have to do accordingly. Basically 3.14 is given when the other inputs of the sum is divisible by 3.14 or 3.14 and the other inputs are multiples or divisible by a common number.