nick13
On a 1-mile test course, a vehicle is travelling at a constant rate of r miles per hour. If it were traveling at a rate of r+20 miles per hour, it would take 4 min less time to travel the test course. What is the value of r?
A. 5
B. 10
C. 20
D. 25
E. 30
Attachment:
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When dealing with multiples of speeds (twice as fast, 20% reduced speed etc), then ratios and weighted average come in handy.
When dealing with number added to speed (s and s + 5 or s - 2 etc) then we must make equations.
\(\frac{1}{r} - \frac{1}{(r+20)} = \frac{4}{60}\)
\(\frac{1}{r} - \frac{1}{(r+20)} = \frac{1}{15}\)
But we can easily find the answer without solving it in the typical quadratic way.
I need 15 in the denominator on RHS so I must have a 3 and a 5 as factors of my denominators on LHS. (anyway not in the options)
If r were 3, I would get a 23 in the denominator of LHS which is not there in the denominator on RHS. Ignore.
If r were 5, I would get a 1/5 and 1/25 so I have a 5 but not a 3. Ignore.
If r were 10, I would get 1/10 and 1/30 - looks promising so check.
\(\frac{1}{10} - \frac{1}{30} = \frac{1}{15}\)
\(\frac{2}{30} = \frac{1}{15}\) -> works
Answer (B)
Note that we must be careful while using this method in Data Sufficiency. It is possible that a quadratic may give us two viable values of x. In problem solving, only one answer is correct so we don't have to worry.