Summand Addition Tree with reduction

When only three summands remain, a 3-2 reduction is performed, followed by addition of the final two summands. (a) How many 7-3 reduction levels are needed to reduce 32 summands to three? Compare this to the seven levels needed to reduce 32 summands to three when using 3-2 reductions. (b) Example 9.3 in Section 9.10 shows that log2k 1 levels of 4-2 reduction are needed to reduce k summands to 2 in a reduction tree. How many levels of 7-3 reduction are needed to reduce k summands to 3?

Example 9.3:

https://www.dropbox.com/s/n4fiopr6a4o7a9s/chegg9.17.png?dl=0