7.1. Three positive numbers were changed as follows: the first was increased by $10 \%$, the second by $13 \%$, and the third was decreased by $20 \%$. Will the product of the three resulting numbers be greater or less than the product of the original numbers?
Answer: less. Solution. Let $a ; b ; c$ be the original numbers. Then $1.1 a ; 1.13 b ; 0.8 c$ are the obtained numbers, and their product ( $(1.1 \cdot 1.13 \cdot 0.8$ ) $a b c=0.9944 a b c<a b c$).