Answer:
36 km/h
Explanation:
The total velocity is the sum of the two velocities that add to the movement of the boat.
Since the wind pushes the boat at 21 km/h and the current that runs in the direction of the movent of the boat is 15 km/h, the total velocity at wich the boat moves is:
21km/h + 15 km/h = 36 km/h
I believe that your answer is true
Answer:
a) 4.94e9 J b) 1.07e10 J
Explanation:
The electric potential energy stored in a capacitor, expressed in terms of the value of the capacitance C, and the voltage between its terminals V, is as follows:

a) For the original capacitor, we can find directly U as follows:
U = 4.94*10⁹ J
b) Prior to find the electric potential energy of the upgraded capacitor, we need to find out the value of the capacitance C of this capacitor, which is identical to the original, except that has a different dielectric constant.
As the capacitance is proportional to the dielectric constant, we can write the following proportion:
ε₂ / ε₁ = 

Once calculated the new value of the capacitance, as V remains the same, we can find the electric potential energy for the upgraded capacitor as follows:

⇒ U = 1.07*10¹⁰ J