$\begin{array}{1 1}(A)\;60\%\\(B)\;50\%\\(C)\;40\%\\(D)\;35\%\end{array} $

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

Step 1:

Let M denote Mathematics,B denote Biology

$\therefore$ P(M)=0.40(40%)

$P(B)=0.30(30\%)$

$P(M \cap B)=0.10(10\%)$

Step 2:

$\therefore P$(M or B)=$P(M\cup B)$

$\Rightarrow P(M)+P(B)-P(M\cap B)$

$\Rightarrow 0.40+0.30-0.10$

$\Rightarrow 0.70-0.10$

$\Rightarrow 0.60=60\%$

Therefore,probability that he will be studying mathematics or biology is 60%

Hence (A) is the correct answer.

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