Question 1062406
Here is the differences
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[ 2 minus 1 ] = {{{ 5.4 }}}
[ 3 minus 2 ] = {{{ 5.4 }}}
[ 4 minus 3 ] = {{{ 5.4 }}} 
so, it's an arithmetic progression
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So, if year number 1 has average bill
{{{ 102.6 }}}, then
Let {{{ n }}} = year number
Let {{{ A }}} = average monthly bill  for 
year number {{{ n }}}
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{{{ A = 5.4n + 102.6 - 5.4 }}}
{{{ A = 5.4n + 97.2 }}}
Now if I start with {{{ n = 1 }}}, I get
{{{ A = 106.2 }}}
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Average monthly bill for year 6 is
{{{ A = 5.4*6 + 97.2 }}}
{{{ A = 32.4 + 97.2 }}}
{{{ A = 129.6 }}}
$129.60