Mental addition with tens and thousands is meant to be a breeze but I had a good laugh at my failure thanks to this cunning video! I couldn't do the last sum and I wasn't the only adult! My children ran up to me as I was laughing: "Mummy what's so funny?". So dc excitedly had a turn each (whilst waiting for their turn in another room), and all three getting the last bit wrong too. Then of course they had the "ooooh.. duh.. haha" moment- as I did before them.
Visit Lets Play Math and have a go- it's for adults too :)