Question

3,5,7,10,11,15,15,20 number series reasoning answer

Answer 1

it goes on alternatively like 3+4=7 then 7+4=11,11+4=15.......
and another series that follows is 5+5=10,10+5=15,15+5=20.........
so, the next number after 20 will be 19.
Hope it helps you !!!