What is the last digit of the expression below?  $$\underbrace{5\times5\times5\times\cdots\times5}_{20  ; 5s} \times \underbrace{7\times7\times7\times\cdots\times7}_{20  ; 7s}$$ 
Overseas Competition->Knowledge Point->Calculation Modules->Whole Numbers->Calculation of Multi-digit Numbers