Two kinds of question here. Firstly doubling whole tens, which should prove straightforward as long as doubling single digits is well established: if double 6 is known to be 12 then doubling 60 to get 120 can be derived quickly.
Secondly, and perhaps much harder, is doubling numbers in the teens. It is unlikely that these will be known, but it is important to establish quick methods of working them out.
Usually this will mean doing three things mentally and once children have to do more than one thing to get the answer then difficulties will arise.
Take doubling 17.
Stage 1: double 7 (which is known) 14
Stage 2: double the tens (which is known) 20
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