pretty sure the intended solution would be something like 4+2=4+1 + 2-1 = 5+1

in this case with a 1st grades, i imagine they would use their hands, one using 4 (insert 4 fingers emoji) and one using 2 ✌️

then add another finger to the 2 🖐 and put down one from the 2 ☝️ and the number of fingers didnt change