abhirup1711 wrote:By what % will the virus population increase in 1 hour?
1.Population triples every 20 min
2. Current population is 100,000

Target question: By what % will the virus population increase in 1 hour?

Statement 1: Population triples every 20 minutes
Let x be population at a certain time.
Then 3x = population after 20 minutes
9x = population after 40 minutes
27x = population after 1 hour

So, the population increases from x to 27x in 1 hour.
The percent increase = 100(27x - x)/x = 2600x/x

IMPORTANT: 2600x/x = 2600 as long as x does not equal zero.
In other words, as long as x (the original population) does not equal zero, then we get a 2600% increase in population.

Now, does statement 1 allow for the original population to be zero?
At this point, I think the question deviates from most GMAT questions, since there's some ambiguity here.
For example, If a certain (fantastic) savings account triples every 20 minutes, does this mean that Frank,s savings account (in the same savings account) increase 2600% in 1 hour?
Well, it depends whether Frank has any money in this account. So, perhaps the population can equal zero.
On the other hand, statement 1 says that that population triples every 20 min, so this seems to suggest that the population was never zero (since a population of zero cannot triple).

Given all of this, I'm inclined to say that the population cannot be zero, which means we get a 2600% increase in population in 1 hours.
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: Current population is 100,000
We know nothing about the rate at which the population increases, so we cannot answer the target question with certainty.
Statement 2 is NOT SUFFICIENT

Answer = A

Cheers,
Brent