"1.5*1.3 does not (even approximately) make 1.05..."
Please read the statement again.
(1 + 0.5) * (1 - 0.3) = 1.05
It's 1 MINUS 0.3. It would then be 1.5 * 0.7 = 1.05
Edit: Also, not that this would be kept anyway, but
than if they were {{tt|additive|1+0.5+0.3=1.9}}.
1 + 0.5 + 0.3 = 1.8, not 1.9. Please check the context and signs before editing formulas in the future. These are incredibly egregious math errors.