Answer:
8.3%
Explanation:
initial cost per house = $10 + $40 + $5 = $55 per house
after the 50% increase in the use of chemicals = $15 + $40 + $5 = $60 per house
new multifactor productivity = new cost of 1 clean house / old cost of one clean house = $55 / $60 = 0.917 or 91.7%
so the change in multifactor productivity = 100% - 91.7% = 8.3%