Question

What is the missing number?

2 3 4 15 12
3 4 5 28 20
4 5 6 45 30
5 6 7 66 42
6 7 8 ?? 56

Answer 1

I have noticed that in the third row, difference between 15 & 28 is 13; difference between 28 & 45 is 17; and difference between 66 & 45 is 21. That means at every stage, the difference is increased by 4. Therefore, the difference between the last two numbers should be 25 and therefore the missing number should be (66+25=) 91.

Answer 2

91 is the solution
(2+3)*3=15
(3+4)*4=28
(4+5)*5=45
(5+6)*6=66
(6+7)*7=91