Let speed of the boat in still water = x miles per hour
Let speed of the current = y miles per hour
When water and current both flow in same direction then effective speed will be sum of both speeds that is (x+y)
now plug the given values in formula speed=distance/time
we get equation:
(x+y)=160/8
or x+y=20...(i)
When water and current both flow in opposite direction then effective speed will be difference of both speeds that is (x-y)
now plug the given values in formula speed=distance/time
we get equation:
(x-y)=160/40
or x-y=4
or x=4+y...(ii)
plug value of x into (i)
4+y+y=20
4+2y=20
2y=16
y=8
plug value of y into (ii)
x=4+8=12
Hence final answer is given by:
Speed of the boat in still water = 12 miles per hour
Speed of the current = 8 miles per hour
ANSWER
D. 25 feet
EXPLANATION
The height of the wall,h, the taut wire and the distance from the base of the pole to the point on the ground, formed a right triangle.
According to the Pythagoras Theorem, the sum of the length of the squares of the two shorter legs equals the square of the hypotenuse.
Let the hypotenuse ( the length of the ) taught wire be,l.
Then
((5x2)+5.5+(6x3)+6.5+(7x5)+(7.5x4)+(8x4))/20
137/20
= 6.85
