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

### 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:
 Julia amenunua kalamu 4 * 3 = <<4*3=12>>12
Dorothy amenunua kalamu 12/2 = <<12/2=6>>6
Jumla ya kalamu zilizonunuliwa ni 12 + 6 + 4 = <<12+6+4=22>>22
Jumla ya gharama ya kalamu ni 22 * $1.50 = $<<22*1.5=33>>33