Ответ:

  1. \(709\cdot23=709\cdot(20+3)=14\,180+2\,127=16\,307\).

  2. \(1360\cdot24=1360\cdot(20+4)=27\,200+5\,440=32\,640\).

  3. \(640\cdot19=640\cdot(20-1)=12\,800-640=12\,160\).

  4. \(3914\cdot38=3914\cdot(30+8)=117\,420+31\,312=148\,732\).

Ответ: 16 307; 32 640; 12 160; 148 732.