Add left to right instead of right to left: add the tens of the second number to the first, then add the remaining units. This is exactly the Hint you'll see in-game if you get stuck.
56 + 21= (56 + 20) + 1= 76 + 1= 77
44 + 38= (44 + 30) + 8= 74 + 8= 82
This one's trickier — the units add up to more than 10.
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Practice 2d + 2d