Students use the count-back method or the relationship between addition and subtraction to find the missing number if it is a minuend or subtrahend. Also, they subtract the numbers to get the difference if the missing number is the difference. There is a lot of practice with this concept in the worksheet subtract 10 within 20. In each problem, the numbers are laid out in the horizontal format. Students should try to use different strategies involving composing and decomposing numbers to solve these problems. This will help them develop flexibility and fluency.