throughput of dma is the data rate of dma. or number of words it can transffer to memory in one seconds.
1 clock time = $\frac{1}{1.2*10^9}$ seconds
16 bytes requires 20 cycles. so 1 byte requires $\frac{20}{16}$ cycles.
so time taken to transffer one byte will be ( $\frac{20}{16}$ cycles.* 1 cycle time) seconds
so in one second we can transfer 960 Mbps.