Answer:
[tex]a_{1}=6[/tex]
Step-by-step explanation:
It is sum of arithmetic progression
[tex]$S_{20}=\frac{20}{2} [2a_{1}+(20-1)\cdot 4]$[/tex]
[tex]$880=10[2a_{1}+(19)\cdot 4]$[/tex]
[tex]$880=10[2a_{1}+76]$[/tex]
[tex]880=20a_{1}+760[/tex]
[tex]120=20a_{1}[/tex]
[tex]a_{1}=6[/tex]
