А) Біріншісінде 25%, ал екіншісінде 30% күміс екі қоспа берілген. 20 кг бірінші қоспаға күміс мөлшері 28% болатындай екінші қоспадан қанша килограмм күмісқосу қажет

Добрый день. Я так понял на экспедицию? Если да, то вот примерное сочинение.
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Hiking TripПоходI like hiking a lot. In my opinion, it’s a great source of pleasure and health benefit. Hiking is very similar to a walking tour or camping. It is a long walk in the countryside with occasional stops. Most people go hiking for pleasure or physical fitness. It is important from time to time to leave crowded cities and go out for a walking tour. The open air has a bracing effect on our minds. The sight of huge trees and green bushes, blooming wild flowers and running streams, the birds’ singing – it all carries us away from the depressing daily routine. I especially like hiking in mountainous areas. It is more thrilling that walking in the plains. Last time we went hiking was in summer. Together with our teacher and a group of school children we went to find some picturesque places on the bank of the River Don. We took all the necessary items, including sleeping bags, some food, first aid kit and mosquito spray. We wore sporty outfit and comfy boots. Our rucksacks were rather heavy, but it didn’t spoil our journey. Most importantly we chose the route of our journey and fully planned the trip. We set out early in morning. Luckily, the weather was perfect. Although, there were some clouds in the sky, the wind soon took them away. Our first meal in the open air felt fantastic. We baked some potatoes and frankfurters. It was rather hot, so we could sunbathe and swim in the river. We also played some outdoor games, such as volleyball and football. The next morning we went fishing. It was a great fun. While hiking, we saw lots of beautiful places. Time passed so quickly and soon we had to go back home. No one wanted to leave. We thought it was an unforgettable experience.

Пусть весь объём работа равен 1, производительность первой бригады равна x, а второй - y. Зная, что работа = производительность●время, получим систему из двух уравнений:
12(x + y) = 1
8(x + y) + 7y = 1 

12x + 12y = 1
8x + 8y + 7y = 1

12x + 12y = 1          |·2
8x + 15y = 1            |·3

24x + 24y = 2
24x + 45y = 3

Вычтем из второго уравнения первое:
24x + 45y - 24x - 24y = 3 - 2
8x + 15y = 1

21y = 1
8x + 15y = 1

y = 1/21
8x + 15·1/21 = 1

y = 1/21
8x = 1 - 5/7

y = 1/21
8x = 2/7

y = 1/21
x = 1/28

время = работа:производительность
Значит, t1 = 1/x = 1/(1/28) = 28 часов.

ответ: 28 часов.