Subtract multiples of 10 from multiples of 10. For example, subtract 20 from 90. Students should think 9 - 2 = 7, so the answer is 70. Subtract multiples of 10 from any number within 100. For example, subtract 30 from 72. Students should think 7 - 3 = 4, so the answer is 42.

Subtract within 100 is not listed until third grade. However, Khan Academy places addition and subtraction within 100, and I have to agree, as third grade is focused on numbers within 1000.

1.NBT.C.6: Subtract multiples of 10 in the range 10-90 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.