The speed of a stream is 3 km per hour. A boat travels upstream 12 km and then returns to its original position downstream along the same route. If the speed of the boat in still water is 9 km per hour, what is the average speed of the boat for the entire round trip?

7 km

Step-by-step explanation:

let the speed of boat in still water is x km/hr. 

given speed of stream is 3 km/hr. 

then speed of boat upstream = ( x - 3) km/hr

speed of boat downstream = (x + 3 ) km/hr 

Distance travelled upstream = 4 km

time taken to travel upstream = distance travelled upstream / speed of boat 

upstream

= 4 / (x - 3 ) (1) 

Distance travelled downstream = 10 km

time taken to travel downstream = distance travelled downstream / speed of boat 

downstream

= 10 / (x + 3) (2)

given that boat takes same time to travel upstream and downstream.i.e; (1) & (2) are equal

i.e. 4/(x-3) = 10/ (x + 3 ) 

4 * (x +3 ) = 10 * (x- 3)

4x + 12 = 10x - 30

12 + 30 = 10x -4x 

42 = 6x 

x = 42 / 6 = 7. 

i.e speed of boat in still water is 7 km/hr.

hey there!

if she's saving for six months, let's divide the total number we need for the trip by six, dividing it into the amount that needs to be saved for each month.

500 ÷ 6 =

she needs to save at least this amount each month, so we should use this symbol: ≥

let x be the amount she saves each month.

your final answer is x ≥

hope this !

step-by-step explanation: