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

### Instruction:
Sarah's age is equal to three times Mark's age minus 4. Mark is four years older than Billy. Billy is half Ana's age. If Ana will be 15 in 3 years, how old is Sarah?

### Response:
 Tuseme Sarah ana umri wa x miaka.
Mark ni x/3-4 = <<x/3-4=x/3-4>>x/3-4 miaka
Billy ni x/3-4-4 = <<x/3-4-4=x/3-8>>x/3-8 miaka
Ana ni x/3-8-4 = <<x/3-8-4=x/3-12>>x/3-12 miaka
Baada ya miaka 3, Ana atakuwa na miaka 15 = x/3-12 + 3 = <<x/3-12+3=x/3-9>>x/3-9 miaka
Ikiwa x/3-9 = 15, basi 15*3-9 = <<15*3-9=42>>42 = x
Sarah ana miaka 42