In class, 30% of the students offered mathematics 20% offered chemistry and 10% offered both.

Given: Math students = 30%

Chemistry Students $=20 \%$

Math \& Chemistry both $=10 \%$

To Find: P(Math or Chemistry)

Now, $\mathrm{P}$ (Math) $=30 \%=\frac{30}{100}=0.30$

$\mathrm{P}($ Chemistry $)=20 \%=\frac{20}{100}=0.20$

$\mathrm{P}($ Math $\cap$ Chemistry $)=10 \%=\frac{10}{100}=0.10$

We know that,

$P(A \cup B)=P(A)+P(B)-P(A \cap B)$

Therefore,

$P($ Math $\cup$ Chemistry $)=0.30+0.20-0.10=0.40$

Hence, number of students studying math or chemistry are 40%