17
[tex]45 \div 3 = 15 \\ 15 \times 4 = 60[/tex]
18
[tex]5350 \div 5 = 1070 \\ 1070 \times 3 = 3210[/tex]
20
[tex]8400 \div 4 = 2100 \\ 8400 \div 2 = 4200 \\ 4200 + 2100 = 6300[/tex]
21
[tex]280 - 15 = 165 \\ 165 \div 5 = 53 \\ 53 \times 3 = 159[/tex]