You actually want it separeted by comma with no space between it, so it takes place in all the minutes you want:
6,21,36,51 * * * * /bin/foo
or you could use it every 15 minutes which would look like this:
6-59/15 * * * * /bin/foo
For more information you can
man crontab 5 or check this url http://www.manpagez.com/man/5/crontab/
The 6/15 doesnt work because it doesnt know how to divide it, if you had it like 0-59/15 it could work but then it would not count the initial 6 that you wish for.
The initial number should be a range that will be divided by the number after the
/ to run every X minutes (where X is the number that comes after the
/) within the range you gave it.
another example would be 0-40/5 which would run every 5 minutes until the 40 minutes.