(8 m) / (1.5 m/s) = (5 and 1/3) second
That number is the least possible time the dog can take to
cross the yard with his walking speed of 1.5 m/s.
The actual length of time it takes him can be any longer time,
but no shorter time.
It depends on the route he follows, and how long and frequent
his stops are.
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Wait ! I take it all back !
We have been led down the Primrose path !
The question can't be answered with only the information given.
We are given the length of the yard.
But in order to "cross" the yard, the dog must traverse its width,
which we do not know.
No answer is possible.