We live in a Base 10 world. This sheet is useful in assisting students to 'add up' to ten. It is a great numeracy skill for kiddos from 2nd grade up...(1st grade as well, but there is too many problems on this sheet). We use this to help us subtract double digit minus single digit problems (i.e. 15 - 9 = ??) The students add up the 9 to 10 with a . Then, there are  more spaces between 10 and 15. Hence, the difference or the number of spaces between 9 and 15 is  +  = 6. This process only works when the it is double digit subtracting a single digit since the difference must pass through 10. This fact needs to be pointed out to students and its physical meaning or they will try and use it for single digit subtracting single digit difference problems (i.e. 8 - 3 = ??) which of course does not make sense.