Question 381669
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Hi,         
Let x represent the charge for 10 boxes
Question sets up the Proportion
{{{ 3/.87 =  10 /x}}}
Cross multiply to solve for x
 3x  = 10*.87
   x = 10*.87/3
   x = $2.90

CHECKING our Answer
  3/.87 = 1/.29 = 10/2.90  = 1/.29