It is basically very conceptual question !

(a)(a+1)(a+2)(a+3)(a+4)(a+5) = 720
(a)(a+1)(a+2)(a+3)(a+4)(a+5)=1X2X3X4X5X6     ( since 1X2X3X4X5X6=720 )

hence a = 1
And consecutive numbers equal to 1,2,3,4,5,6,

actually sir i think the 5 consecutive numbers are 2,3,4,5,6 and not 1,2,3,4,5,6.

:-)