Logic: Problem solving

In my neighborhood, a survey showed that out of two hundred households 70 had a fire extinguisher, 100 had an emergency exit on the roof, and 80 had neither a fire extinguisher nor an emergency exit on the roof.
How many households had a fire extinguisher but no emergency exit on the roof?

A. 40

B. 10

C. 20

D. 30

E. 50

The answer is C. May I know how this is progressed?

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Hi! So we have 200 households with 80 that had neither. So 120 households would have either a fire extinguisher, an emergency exit or both.
Now lets add up the values for fire extinguisher and emergency exit → 70 + 100 which gives 170.

However we can see that it exceeds the maximum value of households with them, so 170-120=50 had BOTH fire extinguisher and emergency exit.
We want to find ONLY fire extinguisher so we subtract 70-50=20.

Therefore the answer is C :)) You can use a Venn diagram to help with this too. Hope this helps!