Question 1013609
{{{ C = .006p^2 - .036p + .7 }}}
{{{ C }}} is plotted on the vertical axis
{{{ p }}} is plotted on the horizontal axis
You need to find the point ( C,p ) where
{{{ C }}} is a minimum
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The formula for {{{ p }}} when {{{ C }}} is a 
minimum is:
{{{ p[min] = -b/(2a) }}} when the equation 
is in the form:
{{{ C = a*p^2 + b*p + c }}}
{{{ a = .006 }}}
{{{ b = -.036 }}}
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{{{ p[min] = -( -.036 ) / ( 2*.006 ) }}}
{{{ p[min] = .036 / .012 }}}
{{{ p[min] = 3 }}}
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making 3 puzzles/day will minimize
the cost/puzzle
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check:
Here's the plot:
{{{ graph( 400, 400, -20, 60, -10, 50, .006x^2 - .036x + .7 ) }}}
Answer could be right.
Plug {{{ p = 3.1 }}} and {{{ p = 2.9 }}} into the
equation and see if {{{ C }}} is NOT a minimum in each case