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Susan drove at an average speed of 30 mph for the first 30 miles of the trip and then at an average speed of 60 mph for the remaining 30 miles of the trip. If she made no stops during the trip , what was Susan’s average speed , in miles per hour, for the entire trip?
A. 35
B. 40
C. 45
D. 50
E. 55
Solution (B):
(Από τις πλέον συνηθισμένες εκφωνήσεις άσκησης ταχυτήτων στο GMAT )
Πρώτο βήμα πάντα βρίσκουμε το συνολικό χρόνο για τη συνολική διαδρομή ,χρησιμοποιώντας συνέχεια ότι
ταχύτητα = απόσταση/χρόνος (u = d/t )
[katex]t_1[/katex] = 30 / 30=1 hour
[katex]t_2[/katex] = 30 / 60 = ½ hour
Oπότε [katex]u_{average}[/katex] = [katex]d_{total}[/katex] / [katex]t_{total}[/katex] [katex]\Rightarrow[/katex]
[katex]U_{av}[/katex] = 60 / 1.5 = 120 / 3 = 40
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