急 ! F5 about Dispersion 13q9

(a)
By calculator, the SD of adjusted marks=20 (correct to nearest integer)
SD of adjusted marks is smaller by a factor of 0.9.

(b)(i)
Ms Tse did not use S=1.1M because the adjusted marks of 92 and 94 will be above 100.
She did not use S=0.8M+15 because the adjusted marks of 92 and 94 are 89 and 90 which are smaller than their original marks.

(b)(ii)
Suggested formula: 0.7M+30