Monthly Water Bills
Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
x1 x2 x3 x4 x5 x6 x7 x8 x9 x10 x11 x12
$40 $42 $40 $38 $48 $50 $58 $62 $56 $46 $44 $44
Write the formula for the mean water bill for the entire year using sigma notation.