U = {Students}

n(U) = 120

M = {Students who passed Mathematics}

n(M) = 90

S = {Students who passed Science}

n(S) = 40

Let x = number of students who passed both subjects.

Covering Mathematics and solving for x.

90 + 40 - x + 5 = 120

135 - x = 120

135 - 120 = x

15 = x

Science only = 40 - x

Science only = 40 - 15

Science only = 25

U = {Students}

n(U) = 120

M = {Students who passed Mathematics}

n(M) = 90

S = {Students who passed Science}

n(S) = 40

Let x = number of students who passed both subjects.

Covering Mathematics and solving for x.

90 + 40 - x + 5 = 120

135 - x = 120

135 - 120 = x

15 = x

Number of samples = 120

Number of students who passed only one subject = 75 + 25 = 100

P(only one subject) = $\frac{100}{120}$ = $\frac{5}{6}$