Xiao Tiantian's mother bought a bag of candy for Xiao Tiantian. Xiao Tiantian eats a part of the candy every day, and eats more sugar every day than the previous day. She ate a total of 31 candies in 5 days. If the fifth day The amount of candy she ate was three times that of the first day, so how much candy can she eat on the fourth day?
小甜甜的妈妈给小甜甜买了一包糖，小甜甜每天都要吃掉一部分糖，每天都会吃掉比前一天多的糖，5天一共吃了31颗糖，若是第五天吃的糖是第一天吃的3倍，那么她第四天能吃多少颗糖．
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