Below is an instruction that describes a task. Write a response that appropriately completes the request in Thai. Please answer in Thai.

### Instruction:
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?

### Response:
 จูเลียซื้อปากกา 4 * 3 = <<4*3=12>>12 ดินสอ
โดรธีซื้อปากกา 12/2 = <<12/2=6>>6 ดินสอ
จำนวนปากกาทั้งหมดที่ซื้อคือ 4 + 12 + 6 = <<4+12+6=22>>22 ดินสอ
ราคารวมของปากกาคือ $1.50 * 22 = $<<1.5*22=33>>33