This question already has an answer here:
Since the childhood days we have been memorizing the tables of numbers saying :
two ones are two (2 x 1 = 2)
two twos are four (2 x 2 = 4)
two threes are six (2 x 3 = 6)....
However recently I came across a rule stating that if the mathematical operation of two or more fixed numbers(constants) yields a fixed numeral, then the singular form of verb should be used to denote the operation connecting the operands and the result. This makes the sentence look like :
Two twos is four.
Is it really so??