$$5+10+15+20+25=\left(1+2+3+4+5\right)\times$$． 
Overseas Competition->Knowledge Point->Calculation Modules->Whole Numbers->Extracting Common Factors from Whole Numbers
$$5+10+15+20+25=1\times 5+2\times 5+\cdots+5\times 5$$  $$=\left(1+2+3+4+5\right)\times 5$$. 