GMAT Club Official Solution:At a professional conference, two optional events were offered: a product demo and a networking breakfast. If 70% of the attendees registered for the product demo and 25% of the attendees registered for both events, how many attendees registered for neither event?{Total} = {Demo only} + {Breakfast only} + {Both} + {Neither}
Also:
{Demo} = {Demo only} + {Both}
So:
{Demo only} = {Demo} - {Both} = 0.70 * {Total} - 0.25 * {Total} = 0.45 * {Total}
The question asks for {Neither}.
(1) 30 attendees registered for the networking breakfast but not for the product demo.
So:
{Breakfast only} = 30
Using the total formula:
{Total} = {Demo only} + {Breakfast only} + {Both} + {Neither}
{Total} = 0.45 * {Total} + 30 + 0.25 * {Total} + {Neither}
{Neither} = 0.30 * {Total} - 30
But {Total} is not known.
Not sufficient.
(2) 150 attendees did not register for the networking breakfast.
Those attendees are in {Demo only} and {Neither}. So:
{Demo only} + {Neither} = 150
0.45 * {Total} + {Neither} = 150
But {Total} is not known.
Not sufficient.
(1)+(2) From statement (1):
{Neither} = 0.30 * {Total} - 30
From statement (2):
0.45 * {Total} + {Neither} = 150
Substitute:
0.45 * {Total} + 0.30 * {Total} - 30 = 150
0.75 * {Total} = 180
{Total} = 240
Therefore:
{Neither} = 0.30 * 240 - 30 = 72 - 30 = 42
Sufficient.
Answer: C.