原式$$=\left( 13+40 \right)+\left( 16+37 \right)+\left( 19+34 \right)+\left( 22+31 \right)+\left( 25+28 \right)$$  $$=53\times 5$$  $$=265$$．  $$(13+40)\times 10\div 2=265$$． 
计算．  $$13+16+19+22+ 25+ 28+31+34+37+40=$$． 

(a) $$286$$ 
(b) $$265$$ 
(c) $$258$$ 