The common difference is -3/4
We can solve this for n
20 - (3/4)(n - 1) < 0
20 - (3/4) n + 3/4 < 0 multiply through by 4
80 - 3n + 3 < 0
83 - 3n < 0
83 < 3n divide both sides by 3
83/3 < n
n > 83/3
n > ( 27 + 2/3 )
The 28th term will be the first negative one
