In a race from point X to point Y and back, Jack averages 30 miles per hour to point Y and 10 miles per hour back to point X. Sandy averages 20 miles per hour in both directions. If Jack and Sandy begin the race at the same time, who will finish first?
2007-11-03 13:49:00 · 6 answers · asked by Anonymous in Science & Mathematics ➔ Mathematics
both will finish at the same time... because:
Jack: 30+10=40, 40/2= 20
Sandy: 20+20=40, 40/2 =20
since d=rt,
you can rearrange it to t=d/r
distance is constant, so is the average rate of X-->Y-->X
and therefore t(Jack)= t(Sandy)
2007-11-03 13:51:55 · answer #1 · answered by Anonymous · 0⤊ 0⤋
Sandy finishes first. Imagine a 60 mile distance each way. Then Sandy takes 3 hours each way; Jack takes 2 + 6 hours.
The reason you can't take Jack's average is that Jack spends more TIME at 10 mph than at 30 mph. If Jack spent the same time at 30 as he did at 10, then he would indeed average 20, but this is not what happened.
2007-11-03 21:02:58 · answer #2 · answered by Facts Matter 7 · 0⤊ 0⤋
Sandy will win.
if the distance is 30 miles from x to y
it will take jack 4 hours
sandy can do it in 3 hours
2007-11-03 20:59:58 · answer #3 · answered by gifted!!!! 2 · 0⤊ 0⤋
sandy
let distance be 60 miles
jack uses 60/30+60/10=8 hours
sandy uses 120/20=6 hours
2007-11-03 20:52:29 · answer #4 · answered by UNIQUE 3 · 0⤊ 0⤋
they will finish at the same time, as their average speeds (for the entire race) are the same: 20 mph.
2007-11-03 20:57:08 · answer #5 · answered by Who, me? 7 · 0⤊ 0⤋
There is a time frame that is left out to get a correct answer, i believe.
2007-11-03 20:53:53 · answer #6 · answered by boilermakersnoopy433 1 · 0⤊ 0⤋