An aircraft company wanted to predict the number of worker-hours necessary to finish the design of a new plane. Relevant explanatory variables were thought to be the plane's top speed, its weight, and the number of parts it had in common with other models built by the company. A sample of 27 of the company's planes was taken, and the following model was estimated:
$$
y_i=\beta_0+\beta_1 x_{1 i}+\beta_2 x_{2 i}+\beta_3 x_{3 i}+\varepsilon_i
$$
where
$y_i=$ design effort, in millions of worker-hours
$x_{1 i}=$ plane's top speed, in miles per hour
$x_{2 i}=$ plane's weight, in tons
$x_{3 i}=$ percentage number of parts in common with other models
The estimated regression coefficients were as follows:
$$
b_0=2 \quad b_1=0.661 \quad b_2=0.065 \quad b_3=-0.018
$$
Interpret these estimates.