The problem is:

In carrying out a survey of the efficiency of the lights, brakes and steering of motor vehicles, 100 vehicles were found defective as follows:

35 defective lights

40 had defective brakes

41 had defective steering

8 had defective lights and brakes

7 had defective lights and steering

6 had defective brakes and steering

a) how many had defective lights, brakes and steering?

b) how many vehicles had defective lights only?

Now I know the answers are 5 and 25 respectively but I cannot form the correct equation to arrive at them.

I think the equation has to be in the form of

x(A u B u C)= x(A) + x(B) + x(C) - x(A n B) - x(A n C) - x(B n C) + (A n B n C)

I would appeciate help with the formulae.

Many thanks.