Bash issue with numbers in specific format : (Need in bash linux)I have a file with numbers like this 

    1.415949602
    91.09582241
    91.12042924
    91.40270349
    91.45625033
    91.70150341
    91.70174342
    91.70660043
    91.70966213
    91.72597066
    91.72876783
    91.73986459
    91.75429779
    91.76781464
    91.77196659
    91.77299733
    abcdefghij
    91.7827827
    91.78288651
    91.7838959
    91.7855
    91.79080605
    91.80103075
    91.8050505

(1)1st I need to remove 91. from all numbers in the list 
(2)then need to delete all numbers which don't have length =10  
(3)then need to delete all numbers which not start with 6 or 7 or 8 or 9 


Any way possible I can do these 3 steps?

