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To get an output Y=1 from the circuit shown, which one of the following sets of inputs will be correct?

Asked in NEET 2012 Mains · Gate combinations and truth tables

Figure: Gate combinations and truth tables
Answer: (2) A=1, B=0, C=1

Step-by-step solution

Idea: collapse the circuit into a single Boolean expression, then test the four sets against it rather than against the drawing.

The first gate is an OR fed by A and B, so the signal leaving it is A+B.

That signal and C are the two inputs of the AND, so Y=(A+B)· C.

For Y=1 both factors must be 1: C must be high, and at least one of A and B must be high.

Only A=1, B=0, C=1 meets both conditions; the other three sets all have C=0.

Why the other options are wrong

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