Answer:
Speed of the river =
km per hour
Step-by-step explanation:
Speed of the boat in still water = 4 km per hour
Let the speed of the river = v km per hour
Speed of the boat upstream = (4 - v) km per hour
Time taken to cover 6 km = 
=
hours
Speed of the boat downstream = (4 + v) km per hour
Time taken to cover 12 km =
hours
Since, time taken by the boat in both the cases is same,

6(4 + v) = 12(4 - v)
24 + 6v = 48 - 12v
12v + 6v = 48 - 24
18v = 24
v = 
v =
km per hour
Knowing the ratio for dogs to cats is 10:9
To get to 50 dogs they multiplied 10x5
So multiply 9x5 as well to get the number of cats
Cats = 45
:) hope this helps
B
Because 2 sqrt5 is = to sqrt 20.
4^2=16, 6^2=36