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Bunuel
David and Stacey are riding bicycles on a flat road at a constant rate. If Stacey is now three miles ahead of David, in how many minutes will Stacey be just two miles ahead of David?

(1) Stacey is traveling at rate of 10 mph and David is traveling at a rate of 12 mph.
(2) 45 minutes ago Stacey was 4.5 miles ahead of David.

Distance between Stacey and David = 3 miles
Required distance between them = 2 miles
David needs to cover 1 mile. Time required to do this will depend only on the relative speed of David.

(1) Stacey is traveling at rate of 10 mph and David is traveling at a rate of 12 mph.

David's speed relative to Stacey in 12 - 10 = 2 mph.
Time taken to cover 1 mile relative distance = 1/2 = 0.5 hrs = 30 mins
Sufficient.

(2) 45 minutes ago Stacey was 4.5 miles ahead of David.
In 45 mins (0.75 hrs), David covered 1.5 miles of relative distance.
So his speed relative to Stacey is 1.5/0.75 = 2 mph (same as given in statement 1)
Sufficient.

Answer (D)
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On what basis is it assumed that David was moving 45 mins ago?

The question also states David and Stacy are riding... nothing about if they both were riding in the past say 45 mins ago.

Couldn't it be the case that Stacy had a head start and David only started to ride say 30 mins ago...
In that case (Sd-Ss)*30 = 4.5...
so Sd-Ss will be different... thus not sufficient

What am i missing? Please help me out here. Thanks :)
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