Tính nhẩm: 13 – 6 =..... 13 – 9 =....... 13 – 7 =...... 15 – 8 =....... 12 – 8 =...... 14 – 7 =...... 18 – 9 =...... 18 – 9 =..... 12 – 8 =....... 11 – 4 =....... 16 – 7 =....... 17 – 9 =.......