Discussion
Certain type of bacteria grows 5% in first hour and decreases in number by 12% in second hour and grows 10% in third hour. Initially 20000 bacteria were present. Find the number of bacteria after 3 hours.
Answer: A
The number of bacteria after three hours is \(20000\times1.05\times0.88\times1.10=20328\), option A.
The other values come from using wrong signs, such as adding the 12% instead of subtracting.
The changes multiply: 20,000 x 1.05 = 21,000; x 0.88 = 18,480; x 1.10 = 20,328. A student who adds the percentages, +5 - 12 + 10 = +3 per cent, would get 20,600, which is not an option, and the other values come from using the wrong factors.
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