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20 G

This is a mission you get during the "For the Republic" mission line for the NCR, while president Kimble visits Hoover Dam. While there may be a couple of ways of completing this, here is the method I used.

Once you are talking to Ranger Grant, ask all question you can of him, make sure you get his itinerary. Then, access the computer terminal next to him(requires science 50), to determine some one has hacked into the system. Now talk to ranger Grant initiate the president event, and while its flying in head to the ladder behind Ranger Grant that leads to the top of The visitor center, where the copter will land. Once it has landed, an engineer will begin fiddling with the plane. examine the plane, and with an explosives or repair of 65 or higher, you will find a bomb. Quickly go tell Ranger Grant, and the speech will be ended(thank god, it is the most boring speech in all of the post apocalypse), Kimble will be hustled to his copter, and the achievement will pop.

Note that there are a lot of "waypoints" on this mission, I imagine there are other methods to complete this, such as possibly catching the sniper, but you would probably still need to remove the bomb anyways, so this is probably the shortest way.