Question

A 780-mile, 7-hour plane trip was flown at two speeds. For the first part of the trip, the average speed was 105 mph.
For the remainder of the trip, the average speed was 120 mph. How long did the plane fly at 105 mph?

Answer 1

the trip is 5 hours, we assume that

the first part it takes x hours at speed 105 mph,

the second part it takes y hours at speed 115 mph.

x + y = 5 that is x = 5- y this is equation 1

105 x + 115 y = 555 this is equation 2

put equation 1 into equation 2, we get

105(5 - y) + 115 y = 555

525 -105 y+ 115 y = 555

y = 3

then x = 5 - y =5 - 3 = 2

so 105 x= 105 * 2 = 210

115 y= 115 *3 = 345

the plan fly 210 mile at speed 105 mph,

the plan fly 345 mile at speed 115 mph.

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