(i) Let `x` is the number of questions that Radhika answered incorrectly.
Then,
`12(3) + (-2)x = 20`
`2x = 16=>x = 8`
(ii)Let `y` is the number of questions that Mohini answered incorrectly.
Then,
`7(3) + (-2)x = -5`
`2x = 26=>x = 13`
(iii)Let `a` is the number of questions that Rakesh answered correctly.
Then, `16-a` is the number that he attempted incorrectly.
Then,
`3a+(-2)(16-a) = 18`
`3a+2a-32 = 18=> 5a = 50=> a = 10`
So, Rakesh answered `10` questions correctly and `6` questions incorrectly.