Question

321:9::512:15::841:40::751:?

Answer 1

321 = 3 * (2 + 1 ) = 9

512 = 5 * ( 1 + 2) = 15

841 = 8 * (4+1) = 40

751 = 7 * (5+1) = 42

Answer 2

321:9
3x (2+1) = 9

512:15
5x (1+2)= 15

841:40
8x (4+1) = 40

751:
7x (5+1)= 42