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

### Instruction:
Karen's class fund contains only $10 and $20 bills,  which amount to $120. The number of $10 bills is twice as many $20 bills. How many $20 bills do they have in their fund?

### 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.