word problem

[pinkdee]

"A commercial Jet can fly from S.F. to Dallas in 3h. A private jet can make the same trip in 3 1/2 h. If the two planes leave from S.F. at noon, how many hours is the private jet twice as far from Dallas as the commercial jet?"


commercial= 2x 3h
private= x 3 1/2h


im having trouble starting off and interprting the problem correctly.

 

[TchrWill]

"A commercial Jet can fly from S.F. to Dallas in 3h. A private jet can make the same trip in 3 1/2 h. If the two planes leave from S.F. at noon, how many hours is the private jet twice as far from Dallas as the commercial jet?"

The distance between the two cities is D.
The speed of the commercial jet is Vcj = D/3.
The speed of the private jet is Vpj = D/3.5.
Then, D - Dt/3.5 = 2(D - Dt/3.
Eliminating D, 1 - t/3.5 = (2 - 2t)/3
Solve for t.

 

[Denis]

Think you made a typo, TchrWill:

"Then, D - Dt/3.5 = 2(D - Dt/3.
Eliminating D, 1 - t/3.5 = (2 - 2t)/3"

Should be: 1 - t/3.5 = 2 - 2t/3 (which leads to t = 21/8)

 

[pinkdee]

thank you 2 very much, i now understand this problem much easier

 

[TchrWill]

"A commercial Jet can fly from S.F. to Dallas in 3h. A private jet can make the same trip in 3 1/2 h. If the two planes leave from S.F. at noon, how many hours is the private jet twice as far from Dallas as the commercial jet?"

Thanks for the heads up Denis. A typo indeed sneaked in.

The distance between the two cities is D.
The speed of the commercial jet is Vcj = D/3.
The speed of the private jet is Vpj = D/3.5.
Then, D - Dt/3.5 = 2(D - Dt/3.
Eliminating D, 1 - t/3.5 = 2 - 2t/3 <----------Correction
Solve for t.