January 2008 SAT Hard Math Question HELP #18

xiggi
Sun, 28 Jul 2013 04:03:39 +0000

WOT, check the stickied thread or search this forum for Xiggi method. I have explained it many times. It is a very simple application of the harmonic mean formula, which itself is derived from the d=rt formula.

Simply stated, the TCB is HOPING student waste time on unneeded steps or mess up with a straight average. Fwiw, the formula is Answer is 2S1S2 / S1+S2.