How to calculate the percentage here?

Question

I have a very very simple question, it relate to physics also

Assuming we have a device with efficiency 85%, how much should be the input power to achieve 100W power in the output?

Here is my problem:

Some people say you have to multiple 100, 1.15 time because there are 15% losses

So we have

100 x 1.15 = 115 W

But some other people say you have to divide 100 by 0.85

So we have

100 / 0.85 = 117.6 W

Please help what is the true way?

Answer 1

$$\frac{Output}{Input}\times 100= \text{efficiency percentage}$$