Ответ:

  1. 456 × 40 = 456 × 4 × 10 = 1 824 × 10 = 18 240.

  2. 870 × 28 = 870 × (20 + 8) = 17 400 + 6 960 = 24 360.

  3. 309 × 50 = 309 × 5 × 10 = 1 545 × 10 = 15 450.

Ответ: 18 240; 24 360; 15 450.