There are $$15$$ balls in a box: white balls, red balls and black balls. The number of white balls is $$7$$ times greater than the number of red balls. How many black balls are there in the box? ($$1998$$ Math Kangaroo Problem, Level $$3-4$$, Question \#$$16$$) 
Overseas Competition->Knowledge Point->Word Problem Modules->Questions Involving Sum, Difference and Multiples->Problems of Sum and Multiple
The numbers of white balls and red balls in total must be a multiple of $7+1=8$, so from $1$ to $15$, only $8$ itself can match the condition. Thus, there are $15-8=7$ black balls in the box. 