Mommy Squirrel goes out every day to harvest some pine nuts. On a sunny day, she can harvest $20$ pine nuts; on a rainy day, she can only harvest $12$ pine nuts. Given that during eight days Mommy Squirrel harvested a total of $112$ pine nuts, how many rainy days were there? 
Overseas Competition->Knowledge Point->Word Problem Modules->Chicken-Rabbit Problems->Solving Chicken-Rabbit Problems by Using Hypothesis