6 - 8 year olds

Iliana has prepared presents for a party and she has set them up inside some boxes. These boxes are numbered from 11 to 99.

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Only ans only where the difference between the tens and units is one there is a present inside. Per example, 87 because 8-7=1. But 78 cant work because you cant substract tens minus units

Would you be able to figure out which are the boxes with presents inside?