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In cycle group if a is generator then it will generate all elements. i.e. a¹⁰= e

If a generator then (a⁸)^x = (a¹⁰) = e

put x= 1,2,3,4,5,6,7,8,.... take smallest x which satisfy this 

(a⁸)⁵ = (a¹⁰) = e

(a¹⁰)⁴ = (a¹⁰) = e

8 also satisfy but order will be smallest such value satisfy the conditionl. hence 5 is answer