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each statement is insufficient. it is given that all prices are changed, so knowing only how 5 prices are changing gives us nothinng about the average.
together we know for sure that the average is going to increase.
the higher prices increase adds to the sum of pricess more than the lower prices decrease takes out. so the overall sum increases (and hence the average).
however - do note that even if you've fallen into the trap of thinking that the average would remain the same... it is still sufficient to answer the question...