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

### Instruction:
 Fedha ya darasa la Karen ina bili za $10 na $20 pekee, ambazo jumla yake ni $120. Idadi ya bili za $10 ni mara mbili ya idadi ya bili za $20. Ni bili ngapi za $20 wanao katika mfuko wao?


### Response:
 Ikiwa n ni idadi ya bili za $20 basi 2n ni idadi ya bili za $10
$10(2n) + $20n = $120
40n = $120
n= $120/40 = <<3=3>>3
Mfuko wa darasa una bili 3 za $20.