Let C be Computer Science, M be Mechanical, and E be Electronics
Total number of students = C + M + E = 240 - (1)
It is also given that C + E = M - (2)
M = Girls in M + Boys in M
Girls in M = $\dfrac{42.5}{100}M$
Boys in M = $\dfrac{57.5}{100}M$
C = Girls in C + Boys in C
Girls in C = $\dfrac{1}{3}\times\ \dfrac{57.5}{100}M\ =\ \dfrac{57.5}{300}M$
Boys in E - Girls in C = Boys in C - Girls in E
Boys in E + Boys in C = Girls in C + Boys in C
E = C - (3)
Girls in E = 28.
From (1), (2) and (3) we get
C + M + E = C + 2C + C = 240
4C = 240
C = E = 60
M = 120
Girls in M = $\dfrac{42.5}{100}M$ = $\dfrac{42.5}{100}\ \times\ 120\ =\ 51$
Girls in C = $\dfrac{57.5}{300}\ \times\ 120\ =\ 23$
Girls in E = 28
The percentage number of girls who opted for Mechanical to the number of students who opted for Electronics is,
$\dfrac{Girls\ in\ M}{E}\ \times\ 100\ =\ \dfrac{51}{60}\ \times\ 100\ =85\%$
The correct answer is option C.