Answer: $84
Step-by-step explanation:
Price of smoke detector in Store A = $17
Price of smoke detector in Store B = $24
Total detectors bought = 12
Price of 12 detectors in Store A = $17 x 12 = $204
Price of 12 detectors in Store B = $24 x 12 = $ 288
Difference = $ 288 -$ 204 = $84
Hence, he will spend $84 less than if he purchased all the smoke detectors at Store B.