The probability that a washing machine will operate satisfactorily for 3 years is 0.82 while the probability that a floor polisher will operate satisfactorily over the same period of time is 0.78. Find the probability that in a 3-year period, only the floor polisher will operate satisfactorily.

Question

I already got, P(A and B) = 0.78(0.82) = 0.6396
I'm not sure if I should subtract it with 0.78 or 0.82

Answer 1

subtraction does not enter the picture

So you want:
Prob(floor polisher ok AND the washing machine not ok)
prob(washing machine not ok) = 1 - .82 = .18

so Prob(floor polisher ok AND the washing machine not ok)
= (.78)(.18)
= .1404

Answer 2

Oh okay.
I see. Thanks a lot!