This question came out in the national examination in my country for 12 year old kids. It raised an uproar among parents as they felt it was too difficult for 12 year olds.
Anyway, I thought it would be interesting to try post it since it involves geometry and GMAT does test geometry afterall.
The question is in the attachment. Enjoy!
Not a bad problem for a 12 year old in Asia. Wish I had received a math education there. Anyway, here's how a 28 year old American would solve it.
Area of large triangle = 15*6=90/2 = 45 subtract the total of the smaller triangles 16 and 4. Answer: 25