1.OA.6 - Subtract multiples of 10 in the range 10-90 from multiples of 10 in the range 10-90, using concrete models or drawings and strategies based on place value, properties of operations, and/or relationship between addition and subtraction.

2.OA.1 - Using addition and subtraction within 100 to solve one-and two-step word problems involving situations of additing to, taking from, putting together, taking apart, and comparing, with unknowns in all positions.