add ten instead, then subtract one (eg: 3+9 =? What is 3 +10? 13. Subtract 1: answer is 12)

subtract one from the other addend, add that to 10 (eg: 9+6=? Move one from the 6, now do 10+5. 10+5 = 9+6 = 15)

Strategy for Teaching the Plus 9 Facts:

Because 9 is close to 10, adding is easy with a little thinking. First, revise with students the place value idea that 10 plus some ones is an easy question, and will be a “teen” number. Introduce students to two strategies:

Add 10, then subtract 1

Subtract 1 from the number other than 9, then add to 10

As the video explains, the main thing is not to teach a required set of steps, or algorithm, but to help students figure out for themselves how 9 being next to 10 makes adding 9 an easy mental process. Ultimately, as with all number facts or times tables, students need to “just know” the answers, having committed them to memory.