Hi Sak32,

ganeshrkamath has properly explained the math behind this question, so I won't rehash that here.

While the GMAT will throw a variety of "standard" formulas/concepts at you, the adaptive nature of the Test means that as you show a consistent ability to answer questions correctly, the program will throw you "non-standard" versions of questions. Sometimes these questions will test your knowledge or rarer formulas or your ability to spot hidden patterns. In this case, the "twist" involves taking a non-standard rate (8 minutes per mile) and converting it into speed (60/8 = 15/2 = 7.5 miles/hour) so that you can answer the question. Pay very careful attention to the information that a prompt gives you; converting information into other formats is a necessary skill to score at a high level in the Quant section.

GMAT assassins aren't born, they're made,
Rich