Aboat going upstream at 4 mph then travels downstream at 10 mph. what is the rate of the boat in still water

7 mph

Step-by-step explanation:

If b is the rate of the boat, and s is the rate of the stream, then:

b − s = 4

b + s = 10

Solve the system of equations with substitution or elimination.  Using elimination, add the equations together:

2b = 14

b = 7

The boat's rate is 7 mph.

reduce the friction between its moving parts

step-by-step explanation:

in fact, by reducing the friction between the moving parts of the machine, it is possible to reduce the energy wasted due to this friction; therefore, more input energy is converted into useful work, and this will improve the efficiency of the machine.

m∠a is 90°

m∠b is 40°