The strategy of doubling and halving is good for high levelled multiplication problems.
e.g. 20 x 97= 10 x 194=1940 . I have halved 10 and doubled 97 to make 10 x 194 (much easier)
Doubling and halving is only good for problems that have a number that can be halved. It is also the same thing for tripling and thirding and quadrupling and quartering etc.