4 friends were singing at the concert. Each song was performed by 3 of them. Person A performed the most --> 10 songs, person B performed the least --> 8 songs. In total, how many songs did friends perform?

Answers: 1) 8; 2) 12; 3) 10; 4) 11.

I've been thinking for some time but I've got no idea how to tackle this problem. Describe your thought process while solving, please.