= Solution
On the same primary-<opcode> expansion interpretation, \b[<opcode>: 9 <bits>; each register field: 8 <bits>]. The printed `rd` is a naming slip: a conventional <MIPS I-type instruction> has fields `rs` and `rt`, with the latter sometimes serving as the destination. There is no separate `rd` field.
Keeping the original 16-<bit> immediate requires $9+8+8+16=\mathbf{41}$ <bits> per instruction. If the <instruction encoding> were held at 32 <bits>, only seven <bits> would remain for the immediate, reducing immediate arithmetic and addressing ranges. A uniform encoding for both enlarged formats would have to accommodate the longer R-type case too; any padding or revised format policy must be specified separately. Register count and instruction width do not determine data-word width.
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