Eroding the rock slowly away
Good Luck :) !
The expression would be $0.20 x f. Once you figure out what the 'f' variable stands for, you have to multiply it by $0.20, and you will have your answer.
By using the equation F=ma (Force = Mass*acceleration), the answer is 15,100 N for the first one and 1.25 m/s^2 for the second.
