Helen sold watermelons at the farmer\textquotesingle s market last weekend. She sold $20$ watermelons on the first day, which is~$\dfrac{1}{3}$~of the number she sold on the second day. On the third day, she sold more after a discount, which is~$\dfrac{5}{3}$~of the number she sold on the second day. How many watermelons did Helen sell altogether? 
Overseas Competition->Knowledge Point->Word Problem Modules->Solving Problems Involving Fractions and Percentages->Finding the Base