Let
[tex]T_1 = a_1,\ a_2,\ \ldots, a_{15}[/tex]
be the 15 questions of the first test, and let
[tex]T_2 = b_1,\ b_2,\ \ldots, b_{15}[/tex]
be the 15 questions of the second test.
If we append the first 5 questions of each test to the bottom of the other, the two tests becomes
[tex]T_1 = a_1,\ a_2,\ \ldots, a_{15},\ b_1,\ b_2,\ b_3,\ b_4,\ b_5[/tex]
[tex]T_2 = b_1,\ b_2,\ \ldots, b_{15},\ a_1,\ a_2,\ a_3,\ a_4,\ a_5[/tex]
So, the tests have 10 questions in common:
[tex]a_1,\ a_2,\ a_3,\ a_4,\ a_5,\ b_1,\ b_2,\ b_3,\ b_4,\ b_5[/tex]
