In Class XI of a school 40% of the students study Mathematics and 30% study Biology.

Let A be the event in which the selected student studies Mathematics and B be the event in which the selected student studies Biology.

Accordingly, $P(A)=40 \%=\frac{40}{100}=\frac{2}{5}$

$P(B)=30 \%=\frac{30}{100}=\frac{3}{10}$

$P(A$ and $B)=10 \%=\frac{10}{100}=\frac{1}{10}$

We know that P(A or B) = P(A) + P(B) – P(A and B)

$\therefore \mathrm{P}(\mathrm{A}$ or $\mathrm{B})=\frac{2}{5}+\frac{3}{10}-\frac{1}{10}=\frac{6}{10}=0.6$

Thus, the probability that the selected student will be studying Mathematics or Biology is 0.6.