Here's a step-by-step approach using the Double Matrix method.

Here, we have a population of kitchen staff, and the two characteristics are:
- can use the meat-cutting machine or cannot use the meat-cutting machine
- can use the bread-slicing machine or cannot use the bread-slicing machine

There are 20 people altogether, so we can set up our diagram as follows:


Given: 17 can use the meat-cutting machine
This means that 3 cannot use the meat-cutting machine
We'll add this to our diagram:


Given: 18 can use the bread-slicing machine
This means that 2 cannot use the bread-slicing machine
We'll add this to our diagram:


Given: 15 can use both machines
We'll add this to our diagram:


Since we know the SUMS of the boxes in each column and in each row, we can complete the diagram as follows:


As we can see, there are 0 people in the box representing someone who cannot use either machine.

So, P( selected person cannot use either machine) = 0/20 = 0

Answer:

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