 #1
songoku
 2,130
 283
 Homework Statement:
 Exam marks, X, have mean 70 and standard deviation 8.7. The marks need to be scaled using the formula Y = aX + b so that the scaled marks, Y, have mean 55 and standard deviation 6.96. Find the values of a and b
 Relevant Equations:

E(aX + b) = a.E(X) + b
Var(aX + b) = a^2 Var (X)
Var (Y) = a^{2} . Var (X)
(6.96)^{2} = a^{2} . (8.7)^{2}
a = ± 0.8
But the answer key states that the value of a is only 0.8
Why a = 0.8 is rejected? Thanks
(6.96)^{2} = a^{2} . (8.7)^{2}
a = ± 0.8
But the answer key states that the value of a is only 0.8
Why a = 0.8 is rejected? Thanks