It's an arithmetic sequence with:
[tex]a_1=2.5\\\\d=5.5-2.5=3[/tex]
The formula of the sum of the n terms of an arithmetic sequence:
[tex]S_n=\dfrac{2a_1+(n-1)d}{2}\cdot n[/tex]
Substitute:
[tex]n=27;\ a_1=2.5;\ d=3\\\\S_{27}=\dfrac{2\cdot2.5+(27-1)\cdot3}{2}\cdot27=\dfrac{5+26\cdot3}{2}\cdot27=1,120.5[/tex]
