Mark buys 6 $2 cans of soup, meaning he spends 6*2= <<6*2=12>>12 dollars on soup.
Mark buys 2 $5 loaves of bread, meaning he spends 2*5= <<2*5=10>>10 dollars on bread.
Mark buys 2 $3 boxes of cereal, meaning he spends 2*3= <<2*3=6>>6 dollars on cereal.
Mark buys 2 $4 gallons of milk, meaning he spends 2*4= <<2*4=8>>8 dollars on milk.
All added together, this means Mark spends 12+10+6+8= <<12+10+6+8=36>>36 dollars
Since Mark only has $10 bills, he spends 36/10= <<36/10=3.6>>3.6 tens on it, which rounds up to 4 as you cannot spend .6 of an undividable item.
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