$B=(1-1/21).(1-1/28).(1-1/36)...(1-1/1326)$
$B=20/21× 27/28× 35/36...1325/1326$
$B=40/42× 54/56× 70/72... 2650/2652$
$B=5.8/6.7 × 6.9/7.8 × 7.10/8.9 ... 50.53/51.52$
$B=5.8/6.7 × 6.9/7.8 × 7.10/8.9 ... 48.52/50.51 × 50.53/51.52$
$B=5.53/7.51$
$B=265/357$