Figure gives a system of logic gates. From the study of truth table it can be found that to produce a high output (1) at R, we must have –

X = 0, Y = 1 X = 1, Y = 1 X = 1, Y = 0 X = 0, Y = 0

X = 1, Y = 0

* Out put R = X $\bar Y$ * If R = 1 it is possible when X = 1 & Y= 0 $Y = \overline{\bar A.\bar B} = A + B$ i.e. OR gate