A group of students go apple-picking. On average, each boy picks $30$ apples and each girl picks $20$ apples. The average number of apples picked by everyone in this groups is $26$. If there are $15$ boys, then how many girls are there in the group? 
Overseas Competition->Knowledge Point->Word Problem Modules->Average Problems 
Every boy picks $$30-26=4$$ more than the average, so boys pick $$4\times 15=60$$ more in total, which is equal to the number that girls picked less than the average in total. Every girl picks $$26-20=6$$ less than the average, so there are $$60\div 6=10$$ girls. 