i) Gravitational force is attractive in nature.
ii) The magnitude of non-contact forces acting on two bodies depend on the distance of separation between them.
Force is inversely proportional to the distance of separation.
That is, the magnitude of force decreases with an increase in separation and force increases as the separation decreases.

Given:
Weight of the boy = force of gravity of the boy = 40 kgf
Time taken by the boy,t_b =  4minutes = 4 x 60 s = 240 s
Weight of the girl= Force of gravity of the girl, F_g = 30 kgf
Time taken by the girl, t_g = 3 minutes = 3 x 60 s = 180 s
Distance covered by both in 30 steps is,
D = 30 x 20 = 600 cm
While climbing upstairs, both have to do work against the force of gravity.
i) Work done by the boy in climbing the stairs is,
      W_b = F x D = 40 kgf x 600 cm
i.e., W_b = 24000 J
Work done by the girl in climbing the stairs is:
W_g = F x D = 30 kgf x 600 cm
W_g = 18000 J
Therefore,

ii) Power developed = 
Boy:
Power developed = 
Girl:
Power developed = 
Therefore, the power developed by them is 1:1

Velocity ratio will not change for the machine of a given design.
It is defined as the ratio of the velocity of effort to the velocity of the load.
Mathematically,
V.R. = 

Let m be the mass of the ice that has to be added.
Heat energy required to melt to lower the temperature = mL = m x 336
Heat energy imparted by the water in fall of its temperature from 40^o C to 0^o C
= mass of water x specific heat capacity x fall in temperature
= 300 x 4.2 x 40^o
If there is no loss of heat,

i) Heat capacity is the amount of heat required to raise the temperature of a body by 1^o C or 1 K.
Thus, by this statement, we mean that the amount of heat that is required to raise the temperature of a body by 1 K is 60 JK^-1.
ii) Specific heat capacity is the amount of heat energy required to raise the temperature of a unit mass of a substance through 1^o C or 1 K. Thus, 130 J kg^-1 K^-1 is the amount of heat energy required to raise the temperature of unit mass of lead through 1 K.