This question is incomplete and the complete question is given in one of the answers.
For the transfer to begin I/O device first need to fill the interface buffer
So the time required = 16MB / 128KB/s = 128s. (In this period of time CPU remains busy doing its own work as it has its own control over the bus).
Now that our interface buffer is full DMA controller takes over the bus and transfer it to the main memory in 28 secs.(Here CPU remains idle as bus is in control of DMA controller).
So % of time CPU is busy = 100*(time period when CPU is busy) / (Total time)
= 100*(128) / (128 + 28)