Data Sufficiency

parulmahajan89 wrote:At a two day seminar, 90 percent of those registered attend the seminar on first day. What percent of those registered did not attend the seminar on either day?

1) Total of 1000 people registered on two day seminar
2) Of those registered, 80 percent did not attend seminar on second day

Dear parulmahajan89,
I'm happy to help with this.

See this blog for info about ratios & proportions:
https://magoosh.com/gmat/2013/gmat-quant ... oportions/
Here's some information about percents:
https://magoosh.com/gmat/2013/gmat-quant ... -percents/

The short answer is: nothing is sufficient here.

The number of people is irrelevant --- all the other information is in percents, and we are asked a prompt question about percent, so the number of the people is completely irrelevant to the calculation.

With all combined information, we know
First day --- 90% attended, so 10% didn't
Second day --- 80% didn't attend, so 20% did.

It could be that the 10% that didn't attend the first day was part of the 80% that didn't attend the second day, so this 10% didn't attend either day.
OR, it could be that everyone in that 10% that didn't attend the first day felt guilty, and were part of the 20% that attended the second day, so that everyone attended at least one day.

Even given all the information, we cannot determine anything definitively. Altogether, everything is still insufficient. Answer = (E).

Does all this make sense?
Mike

parulmahajan89 wrote:Hi Mike,
So total number of 1000 people is irrelevant here?

Thank you
Parul

Dear Parul,
Yes, this is a tricky thing about ratio/percent problems. We can solve a percent or ratio problem just with information about percents or ratios, without knowing any of the numerical values. If we have two numerical values, we can use that to calculate one of the percents or ratios, and that would add to what we know. BUT, if we have only one numerical value, we can use that, by itself, to calculate any new percents or ratios, so it doesn't add anything to the problem.
Does this make sense?
Mike