一、口算。
$530 - 70 =$ $420 \div 30 =$ $180 \times 5 =$
$660 \div 60 =$ $39 + 14 =$ $850 \div 50 =$
$24 \times 50 =$ $280 \div 40 =$ $48 \times 20 =$
$33 \div 11 =$ $28 \times 50 =$ $540 \div 9 =$
$80 - 21 =$ $6 \times 800 =$ $37 + 58 =$
$420 \div 70 =$ $480 \div 40 =$ $180 \div 30 =$
$24 + 57 =$ $230 + 680 =$ $91 - 37 =$
$270 \div 90 =$ $36 \times 20 =$ $750 \div 50 =$
$30 \times 27 =$ $720 \div 3 =$ $730 - 380 =$
$560 \div 7 =$ $360 + 240 =$ $66 \times 3 =$

460 14 900
11 53 17
1200 7 960
3 1400 60
59 4800 95
6 12 6
81 910 54
3 720 15
810 240 350
80 600 198

二、计算下面各题。
$(18 \times 79 - 19) \div 23$ $850 \div (73 - 39) \times 48$ $972 \div [18 \times (321 - 318)]$

61 1200 18