Answer:
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First of all, 2^n and 3^n are exponentials with different bases, and thus their sum cannot be simplified beyond 2^n + 3^n. In other words, these two functions cannot be combined ino one function (such as 4^n).
You may gain much more insight by graphing 2^n, 3^n and 4^n to determine whether there is truth in the given statement or not.
Let 
be the number of items. Each takes 1/8 hour to purchase, so a total time of 
. If we have 3/4 hour, then
That's part a.
b. Multiplying by 8,
c. We have time to by 6 items at 1/8 hour each in 3/4 hours.
xy = 30
x - y = 1
x + y = 11
First, we need to consider factors of 30:
1, 2, 3, 5, 6, 10, 15, 30
Which two of these, subtracted, would equal 1?
5 and 6
6 x 5 = 30
6 - 5 = 1
6 + 5 = 11
