Answer
${$120}$, ${$40}$ lower than before.
The efficiency has increased by $33.33\%$ in cars-per-dollar.
Work Step by Step
The cost to pay a salesperson for 8 hours is now ${$240}$, and now sells two cars in that time, so the labor cost per car sold is ${$120}$, which is ${$40}$ lower than before.
Efficiency (cars per dollar):
At $\$20$:
$\frac{1}{160}=0.00625$ cars per dollar
At $\$30$:
$\frac{2}{240}\approx 0.00833$ cars per dollar.
Efficiency increased by
$\frac{0.00833-0.00625}{0.00625}=0.3333=33.33\%$
So the efficiency increased by about $33.33\%$ in cars-per-dollar.
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