Answer :
The equation that should be used is
$85 = f + 6% + 18%
6% = $5.10
18% = $14.40
$5.10 + $14.40 = $19.50
$85 - $19.50 = 65.50
The cost of the food would have been around $65.50
$85 = f + 6% + 18%
6% = $5.10
18% = $14.40
$5.10 + $14.40 = $19.50
$85 - $19.50 = 65.50
The cost of the food would have been around $65.50