Why do we add 6 in BCD addition?
Four binary digits count up to 15 (1111) but in BCD we only use the representations up to 9 (1001). The difference between 15 and 9 is 6.  If you want 9+1 to produce 10, which is 1 0000, you have to add 6 to make 1010 wrap to 1 0000.

If you're adding minutes, you similarly add 40 to a time which exceeds 59 minutes. Example: 45 minutes plus 35 minutes is 80 minutes. Correction, add 40 to make 120. Now insert a colon: 1:20. One hour, twenty minutes. 40 is the difference between 100 and 1:00.