Dorothy, Julia, and Robert go to the store to buy school supplies. Dorothy buys half as many pens as Julia. Julia buys three times as many pens as Robert. Robert buys 4 pens. If one pen costs $1.50, how much money did the three friends spend in total on pens?
Julia buys 4 * 3 = <<4*3=12>>12 pens
Dorothy buys 12/2 = <<12/2=6>>6 pens
The total number of pens purchased is 4 + 12 + 6 = <<4+12+6=22>>22 pens
The total cost of the pens is 22 * $1.50 = $<<22*1.5=33>>33
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