The 7421 IC is composed of two independent Four-Input AND gates.
The truth table for the 4-input of the AND is identical to that of the 2-Input variant. Naming the logical inputs and outputs as zeros and ones allows for easy calculation of the function performed.
Let inputs be marked as A 'B' C and D. The output will be called X.
Then, A ' B ' C ' D = X. For example, if 2 of the inputs were logical one and two of them were logical zero, then 0'0'1'1 = 0, therefore the output of the gate would be zero.
By a simpler method, simply consider the fact that the gate will only be output logical one if and only if ALL inputs are logical one (HIGH).
