(i) For country A, the median lies between 35 and 50 marks because \(159 < 150\) (half of \(300\)), so the median is less than \(35\).
For country B, since \(144 < 150\) and the next cumulative frequency is at \(198 > 150\), the median lies between \(50\) and \(70\). Therefore, the median for country B is higher than that for country A.

(ii) Number of candidates in country A who scored between 20 and 34 marks: \[ 159 - 68 = 91. \]

(iii) To estimate the mean mark for country A, use class midpoints: \(4.5,\ 14.5,\ 27,\ 42.5,\ 60,\ 85\). \[ \text{Mean} \approx \frac{(4.5 \times 25) + (14.5 \times 43) + (27 \times 91) + (42.5 \times 75) + (60 \times 26) + (85 \times 40)}{300} = 37.6. \] So, the estimated mean mark is approximately \(37.6\).