К÷40=1680×30
к÷40=50400
к=50400×40
к=201600
201600÷40=1680×30
(201600÷40)÷30=1680
1680=1680
m×20=489000-410860
m×20=78140
m=78140÷20
m=3907
3907×20=489000-410860
410860+3907×20=489000
489000=489000
75880÷(n-29965)=70
n-29965=75880÷70
n-29965=1084
n=1084+29965
n=31049
75880÷(31049-29965)=70
70=70