Because we know that the number of 15s and 29s are positive integers (we can't buy portions of a stamp), we can't use the "n unknowns, n equations" rule. (And, if we think like the test-maker who is trying to catch those over-eager to use the rule, we may even anticipate that the restriction imposed by the integer status of the number of 15s and 29s means that we will likely need fewer than "n" (likely "n-1") equations to find any of the "n" unknowns).
(1): Because 15 and 29 aren't factors of 440, we know we need some combination of both to reach 440.
Because 15 + 29 is a combination--44--that is clearly a factor of 440, we know that the required combination of 15 and 29s is 1:1. Because clearly ten (15+29)s will give us 440, (1) is telling us that Joanna bought ten $0.15 stamps.
(2): Because we know nothing about the total value, this information is unhelpful in answering the question.
[spoiler](A)[/spoiler]
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