Hi Chinn_asama,

The phrases "at best" and "at worst" are usually meant to define an "extreme" possibility. When using these phrases, it's important to think about the "intent" of what the sentence is trying to communicate:

The sentence "It is, AT BEST, one hour's work for a D grade computer expert." does NOT sound correct, because the sentence appears to define the FASTEST that an expert could work, but doesn't define what is likely to happen.

The sentence "It is, AT WORST, one hour's work for a D grade computer expert." makes more sense, in that it defines the worst case scenario (one hour of work), but implies that the expert could probably do the job faster.

GMAT assassins aren't born, they're made,
Rich