Usually the easiest way of doing these is to count up from the smaller number.
These look complicated but are easy to do 'in your head' if you count up.
Not quite so easy. Don't forget you can get a printout of questions and answers when 10 have been completed. Great for short term assessment.
Rearrange the question slightly to make this easier. Ten out of ten brings a fun on-screen reward.
Usually the best way to do these 'in your head' is to subtract the nearest multiple of ten and adjust.
Just 60 seconds to get as many right as possible. Don't let the decimal point confuse you.