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step 1: a b c x a b c x a b c x a b c x a b c x a b c x a b c x...
step 2:       d       e       f       x       x       x       d...

seconds task   
  4     a b c    
...
144     h i j  run one of them every other group 
               (pattern repeats after 6 groups)
600     k      run once every 25th group

1   a
2   b
...
22  b
23  c
24   potentially run one of the every 600 second group