 Solution

Let $s_i$ be sunny days and $l_i$ latitude. The R linear-model formula expands to a normal linear model with both main effects and their interaction term:The fitted conditional mean isThe residual standard error is $0.2836$, giving the usual variance estimate $s^2=0.08042896$, and $n=96+4=100$.

The estimated interaction term is tiny relative to its standard error: $t=-0.103$ and the two-sided p-value is $0.91811$. A sensible initial simplification is to remove the interaction and refit an additive linear model, lm(energy ~ sunny + latitude). Both main effects are significant in the supplied fit, and should initially be retained. Do not simply copy their old coefficient estimates into the new fit: removing a design column generally changes all estimates. Centering predictors makes intercepts and main-effect interpretations more useful, but does not change the fitted surface. The Box–Cox transformation in part (d) suggests a further response-scale improvement; after changing the response scale, reassess the interaction and diagnostics.