The question asks for the Dorbish-Bowley Price Index, \(P_{01}(D-B)\), given Laspeyres' Price Index, \(P_{01}(L)\), and Paasche's Price Index, \(P_{01}(P)\).
The formula for the Dorbish-Bowley index is the arithmetic mean of the Laspeyres and Paasche indices:
\[ P_{01}(D-B) = \frac{P_{01}(L) + P_{01}(P)}{2} \]
Given:
\(P_{01}(L) = 90\)
\(P_{01}(P) = 40\)
Substitute the values into the formula:
\[ P_{01}(D-B) = \frac{90 + 40}{2} = \frac{130}{2} = 65 \]
Therefore, the Dorbish-Bowley Price Index is 65.