642X 123369/321963
=642x(123*1003)/(321*1003)
=642*123/321
=642/321*123
=246
9/6-7/12+9/20-11/30+13/42
=9/6-(1/3+1/4)+(1/4+1/5)-(1/5+1/6)+(1/6+1/7)
=9/6-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7
=9/6-1/3+1/7
=55/42