[ig1]

Generate two random numbers with your rand5 function, let them be the two digits of your base 5 number. Each number from 0-24 has a 1/25 chance of being picked.

If the number is greater than 6 throw it away and repeat, otherwise you have your random number.

[llimllib]

Works, but not as efficient as possible. Look at patio11's solution; you only need to throw it away and repeat if it's >= 21. Otherwise you can just take n mod 7.