Question

1=5,2=6,3=8,4=11,5=?

Answer 1

1=5
2=5+1 =6
3= 6+2=8
4=8+3 =11
5=11+4 =15

Answer 2

1 = 5
2 = 6     {5+1}
3 = 8     {6+2}
4 = 11   {8+3}
5 = 15   {11+4}

so the answer is 15