As we know that work done is area under F-s curve.
(i) Work wond at the end of 10m `W=A_(1)` (Area from 0 to 10) `=1/210 10J =50J`
(ii)Work done at the end of 20 m `W=A_(1)+A_(2)` (Area from 0 to 20 m) `=50+50=100J`
(iii) Work done at the end of 30 m `W=A_(1)+A_(2)+A_(3)` (Area from 0 to 30m)
`=50+50-(A_(3)=-50)=50J`