Which is the correct method for increasing a number by a Percentage?

I will also explain how we got the formula [a number/a percentage] for calculating the cycle time of an operation at a percentage of efficiency.
I will take the above example for explaining this.
@ 100% efficiency the cycle time of an operation is 9 minutes and the work done to be done in one cycle is W.
Now we got an equation 1
W/9=w1
Here w1 is the work done in a unit time at 100% efficiency.
Like this we will get another equation, equation 2
W/x=w2
Here w2 is the work done in a unit time at 85%efficiency
And x is what we needed, cycle time at 85% efficiency.
Since x is cycle time at 85% efficiency, w2=w1*85%
The third equation.
By solving these three equations we get the result
x=9/85%