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If we are given that COD is 60 degrees, then angle COA must be 120 degrees (180 - 60). This does not help us with angle BAO that we are trying to find.

Insufficient. Thus, eliminate A & D.

2. The degree measure of angle BCO is 40.

We are given BCO, but this measure alone does not help us with finding BAO. Thus eliminate B.

Trying both together, in triangle ACO, in order to find one angle measurement, you would need 2 angles out of 3. Combining both 1 and 2 together, we are given that. By being provided the measurement of angle COD in 1, you can find COA as 120 degrees. Further, in 2 you are given the measure of BCO, which is 40. Combining the two, the result is 160 degrees (120 + 40 = 160). Subtracting 160 from 180 will give you 20 degrees as the measurement for angle CAO, thus providing enough info. to solve the problem.

Could you explain the way in which you assume that the line CBA is a straight line? That's the only point I miss

GMAT gives you the figure and at this figure CBA is a line. It is so unusual maybe even unreal that GMAT uses such geometric illusion At least I've never seen such type of ambiguity in real problems. _________________

Could you explain the way in which you assume that the line CBA is a straight line? That's the only point I miss

GMAT gives you the figure and at this figure CBA is a line. It is so unusual maybe even unreal that GMAT uses such geometric illusion At least I've never seen such type of ambiguity in real problems.