The volume of rain that fells in the field is simply given by the area of the field, which is

multiplied by the height of rain that fell, which is

Therefore, the volume is

3 protons should be your answer
Look at bitesizes physics section, they have all the information you need to complete this question.
Assuming it is on a horizontal surface:
friction = μR
R = 20g (g is gravity 9.81)
so Friction = 0.085 x 20g
Work done is force x distance
so Work done = 0.085 x 20g x 28
= 466.956 J