Standard(s):
[MA2019] (1) 14 : 14. Given a two-digit number, mentally find 10 more or 10 less than the number without having to count, and explain the reasoning used.
[MA2019] (1) 15 : 15. Subtract multiples of 10 from multiples of 10 in the range 10-90 (positive or zero differences), using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction. Relate the strategy to a written method and explain the reasoning used.
