Doubling multiples of ten mentally. Lots of different strategies explained via the Help link.
Doubling multiples of 100 is very similar to doubling multiples of 10. Different ways to approach this are found via the Help link.
Knowledge of how to halve 2-digit even numbers comes in handy with these larger multiples of 10.
Doubling decimal fractions. Don't forget the decimal point.
Use skills learned when halving 2-digit numbers when trying these. Remember there is a printout page after 10 questions.
Just 60 seconds to halve as many multiples of 10 as you can. Good luck!