This is also taught in second grade in the US. Edit: https://www.khanacademy.org/math/cc-2nd-grade-math/cc-2nd-add-subtract-100/cc-2nd-add-sub-miss-value-prob

This is also taught in second grade in the US. Edit: https://www.khanacademy.org/math/cc-2nd-grade-math/cc-2nd-add-subtract-100/cc-2nd-add-sub-miss-value-prob/v/missing-numbers-in-addition-and-subtraction-within-100?modal=1 https://www.khanacademy.org/math/cc-2nd-grade-math/cc-2nd-ad...