Question

The Smiths are moving across the country. Mr. Smith leaves 5 hours before Mrs. Smith. If he averages 75mph and she averages 95mph, how many hours will it take Mrs. Smith to catch up to Mr. Smith?

Answer 1

How much of a head start in miles did Mr. Smith have?

+d%5B1%5D+=+60%2A4+

+d%5B1%5D+=+240+ mi

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Start a stopwatch when Mrs. Smith leaves. Let +t+

= the time on the stopwatch in hrs when Mrs. Smith

catches up with Mr. Smith.

Let +d+ = the distance in miles that Mrs. Smith

travels until she overtakes Mr. Smith

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Mr. Smith's equation:

(1) +d+-+240+=+60t+

Mrs. Smith's equation:

(2) +d+=+80t+

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Substitute (2) into (1)

(1) +80t+-+240+=+60t+

(1) +20t+=+240+

(1) +t+=+12+

Mrs. Smith overtakes Mr. Smith in 12 hrs

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check:

(2) +d+=+80t+

(2) +d+=+80%2A12+

(2) +d+=+960+ mi

and

(1) +d+-+240+=+60t+

(1) +d+-+240+=+60%2A12+

(1) +d+=+240+%2B+720+

(1) +d+=+960+ mi