210÷42×6,等于210÷7,最后等于30
210÷42×6=210÷7÷6×6=210÷7=30
=210*6/42=30*7*6/42=30
210÷42×6=210÷(6×7)×6=30÷6×6=6÷6×30=30
210÷42ⅹ6=30ⅹ7ⅹ6÷42=30ⅹ42÷42=30