There are 3 sections in a question paper and each section contains 5 questions. A candidate has to answer a total of 5 questions, choosing at least one question from each section. Then the number of ways, in which the candidate can choose the questions, is:

$\begin{array}{lcc}A\left(5\right) & B\left(5\right)& C\left(5\right) \\ 1& 1& 3\\ 1& 3& 1\\ 3& 1& 1\\ 2& 2& 1\\ 2& 1& 2\\ 1& 2& 2\end{array}$

Number of ways to choose,

(1,1,3) or (1,3,1) or (3,1,1) questions from A, B, C = $\left({}^5{C}_1 x {}^5{C}_1 x {}^5{C}_3\right) x 3 =750 ways$

(2,2,1) or (2,1,2) or (1,2,2) questions from A, B, C = $\left({}^5{C}_2 x {}^5{C}_2 x {}^5{C}_1\right) x 3 = 1500 ways$

Hence, the total number ways = 2250