大牛，求助JJ中的105题，看不懂啥意思？

1.        In a certain group of children, each children either has 3 pens and 1 pencil or has 1 pen and 2 pencils, if the average arithmetic mean of pencil is 1.6, what is the arithmetic mean of pen?

答案1.8

请问题目说得是啥意思呢？看不太懂，请教大牛