2B. An alloy of zinc and silver with a mass of 3.5 kilograms contains $75\%$ silver. When this alloy is melted and mixed with another alloy of the same elements, a third alloy with a mass of 10.5 kilograms is obtained, which has $84\%$ silver. What is the percentage of silver in the second alloy?
Solution. In the first alloy, there was $3.5 \cdot 0.75 \mathrm{~kg}=2.625 \mathrm{~kg}$ of silver. In the third alloy, there was $10.5 \cdot 0.84 \mathrm{~kg}=8.82 \mathrm{~kg}$ of silver. To the first alloy, $(10.5-3.5) \mathrm{kg}=7 \mathrm{~kg}$ of alloy was added, which contained $(8.82-2.66) \mathrm{kg}=6.16 \mathrm{~kg}$ of silver. Therefore, in the second alloy, there was $\frac{6.195 \cdot 100}{7}=88 \%$ silver.

3AB. Igor, Jovan, Kire, and Lazar invited their girlfriends to a dance. During one dance, Betty danced with Igor, Ana with the boy of Vера, Gordana with the boy of Ana, Jovan with the girl of Kire, and Kire with the girl of Igor. Determine the couples in love.