12. a) 2 × 8:4, 14: 7 × 9, 5×4:10

a)

$$2 \times 8 : 4 = 16 : 4 = 4$$

$$14 : 7 \times 9 = 2 \times 9 = 18$$

$$5 \times 4 : 10 = 20 : 10 = 2$$