Very Confusing GMATPrep Iscosceles Triangle Problem

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gmat8000: Senior | Next Rank: 100 Posts; Posts: 40; Joined: Sun Oct 21, 2007 4:58 pm; Thanked: 3 times

by gmat8000 » Sun Oct 21, 2007 5:22 pm

I bet the solution for this is easy. Please explain.

In isosceles triangle RST, what is the measure of angle R?

1) The measure of angle T is 100
2) The measure of angel S is 40

The answer is A. WHY???

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Source: Beat The GMAT — Data Sufficiency | Sun Oct 21, 2007 5:22 pm

cleanbold058: Newbie | Next Rank: 10 Posts; Posts: 8; Joined: Fri Sep 28, 2007 5:39 am; Thanked: 1 times

by cleanbold058 » Sun Oct 21, 2007 5:56 pm

Well the sum of three angles of a triangle is 180 degrees. So the other two angles must add upto 80 degrees. Since it is an isosceles triangle, two of its angles must be equal. So the remaining two angles add upto 80 and are equal so each must be 40 degrees. Hence A is sufficient.

B just lists one angle is 40 degrees. Now the other two could be 70 each or one angle could be 40 and the other 100. So B is not sufficient.

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