I have a slightly different approach to this exercise:
The final monthly mean has to be 6,000 (200% of 2,000) for the 4-year period, so plugging in the meanformula:
\(Mean =\frac{2,000*12 + x*36}{48}=6000\)
And they are asking for the number of units sold during the last 3-year period in order to get to that mean value, the asked value therefore is\(x*36\).
\(6,000*48 = 24,000+36*x\)
\(36*x=6*10^3*48-24*10^3\)
\(36*x=24*(12*10^3-10^3)\)
\(36*x=24*(11*10^3)\)
\(36*x=264*10^3\) --> Answer D
...
The final monthly mean has to be 6,000 (200% of 2,000) for the 4-year period, so plugging in the meanformula:
\(Mean =\frac{2,000*12 + x*36}{48}=6000\)
And they are asking for the number of units sold during the last 3-year period in order to get to that mean value, the asked value therefore is\(x*36\).
\(6,000*48 = 24,000+36*x\)
\(36*x=6*10^3*48-24*10^3\)
\(36*x=24*(12*10^3-10^3)\)
\(36*x=24*(11*10^3)\)
\(36*x=264*10^3\) --> Answer D
...
