Line up 60 boxes in order, each representing a time slot for one instruction. Selecting any 30 of them would represent one way the 60 instructions could be interleaved, if those 30 selections represent the first loop. A different selection of 30 boxes for the first loop would represent a different arrangement for the 60 instructions, and all arrangements may be obtained this way. So 60 choose 30 would be all possible arrangements.