The speed to his sister house is 50mph.
The speed of an object is the magnitude of the rate of change of its position with time or the magnitude of the change of its position per unit of time; it is thus a scalar quantity.
Here, let the speed of his sisters house be x mph.
then, 3x = 2.5(x+10)
3x = 2.5x + 25
3x - 2.5x = 25
0.5x = 25
x = 25 / 0.5
x = 50
Thus, The speed to his sister house is 50mph.
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