In the first grade, students subtract a multiple of 10 from another multiple of 10. Building on it, we start off by with the subtraction of a multiple of 10 from a 2 digit number. 
Understanding of place value comes in handy, as this is the same as decreasing the amount of 10's in a number. Subtract Tens from a 2 Digit Number Worksheet has problems on subtracting using models (base 10 blocks) followed by problems involving direct subtraction.