Logical Reasoning :: Number Series - Discussion

Difference in alternative numbers 

here 1,3,5............... terms are decreasing by 10

and 2,4,6,8................ are being increased by 5

so 6th term = 15 5=20;

starting from the first number, every alternate number is formed by Reducing 10. And from the second number, every alternate number increases by 5. 

there is alternate series going on, starting decending order from 80 in difference of 10.