Please help me understand this average speed problem:

shibsriz@gmail.com wrote:Please help me understand this average speed problem: 320 miles were run @80 mph, this took him 4 hours. 200 percent longer than 4 hours is 4 + 8 = 12 hours, hence the average speed in the entire trip = total distance/total time = 640/(4 + 12) = 640/16 = [spoiler]40 mph Take C[/spoiler]

But where do we use the formula: d(total distance)/(d1/s1+d2/s)? Thanks sanju09 wrote:shibsriz@gmail.com wrote:Please help me understand this average speed problem: 320 miles were run @80 mph, this took him 4 hours. 200 percent longer than 4 hours is 4 + 8 = 12 hours, hence the average speed in the entire trip = total distance/total time = 640/(4 + 12) = 640/16 = [spoiler]40 mph Take C[/spoiler]

can someone explain me why 200% longer is equal to 12h and not 8h?

Let t� = the time for the first 320 miles and t₂ = the time for the second 320 miles. Since the first half of the trip is traveled at 80mph, t� = d/r = 320/80 = 4 hours. Since Mike takes 200% LONGER to travel the remaining 320 miles, t₂ = t� + 200% of t� = 4 + (200/100)(4) = 4 + 8 = 12. Average speed = (total distance)/(total time) = 640/(4+12) = 40mph. The correct answer is C.

Hi confused13, Many Test Takers mistake a "200% increase" with "doubling" - these two ideas are NOT the same. Here's an example of why: Let's say you have $10. If you increase that by 100%, then you have 10 + 10 = $20. So increasing by 100%…. DOUBLES your money If you increase that by 200%, then you have 10 + 20 = $30. So increasing by 200%….TRIPLES your money If you increase that by 300%, then you have 10 + 30 = $40. So increasing by 300%….QUADRUPLES your money Etc. This prompt mentions 200% longer (time). We start with 4 hours. An increase of 100% = 4 + 4 = 8 hours An increase of 200% = 4 + 8 = 12 hours Since the two "legs" of the trip take 4 hours and 12 hours, respectively, the total time = 4 + 12 = 16 hours. GMAT assassins aren't born, they're made, Rich

I'd like to add to Rich's post and say that SOMETIMES 200% does mean "twice" or "doubled." This occurs when the word "of" is used in conjunction with 200%. For example, if we say that y is 200% OF x, we are saying that y = 2x. In other words, y equals twice x. Likewise, if we say that y is 300% OF x, we are saying that y = 3x. In other words, y equals three times x. Cheers, Brent