Equation involving % yield

Please can somebody help with the following question?

I know the % yield involved with a process, but need a formula which will calculate the total input required to get to a set output.

The formula needs to be dynamic so it can recalculate based on any combination of output and yield.

As an example.

Wholehead broccoli is converted to floretted broccoli.

If the floretting process has a 70% yield, what is the total amount of wholehead broccoli needed, to get to 1tonne of floretted broccoli?

I'm going to write this formula in excel but would like to understand the maths behind it first.

Thanks for any help.

Phil