Tap/Click an (?) answer box, then either:
1) Enter your answer via tap/click or the keyboard (black = correct; red = incorrect), or
2) Repeat tap/click of the (?) box to see the correct digit, shown in green. (Use repetitive tap/click to facilitate left to right mental computations.)
This web app is free to use for educational purposes.
© 2019 T.F. Fletcher (fletc003@gmail.com )