My statistics book says there are 8 degrees of freedom for this data set. i don't understand how they got that number. # of books education level 68 11 345 15 276 16 756 12 43 6 546 14 58 6 187 14 286 9 93 8 376 11 623 18 876 12 28 4 289 15

It's a bit unclear with the table this way, but I count fifteen points, fifteen lines of the table, each a pair of numbers.

That's 15 degrees of freedom in the data.   When modeling, each parameter in the model uses up one degree of freedom, so you'd use a smaller number of degrees of freedom when calculating t statistics, etc.

a28=58

step-by-step explanation:

a1=a1+(n-1)d

a28= 4+ (28-1)2

a28=4+(27)(2)

a28=4+54

a28=58