Drag and drop the number from the right to the number on the left so they both add up to 20.
In other words: Click and hold the button down while moving the number. Release the button when over the other number.
The resources listed below are aligned to the same standard, (2OA02) re: Common Core Standards For Mathematics as the Addition and subtraction game shown above.
Fluently add and subtract within 20 using mental strategies.2 By end of Grade 2, know from memory all sums of two one-digit numbers.