Ask and answer using for or since. 1 The ground is wet. (rain) > A: Has it been raining?

2 A: Have you been working hard? (Ты работаешь упорно?)
B: Yes, since this morning./Yes, for two hours. (Да, с утра/Да, два часа)
3 A: Have you been playing in the garden? (Ты играешь в саду?)
B: Yes, for two hours./Yes, since this morning. (Да, два часа/Да, с утра)
4 A: Have you been sitting in the sun? (Ты сидишь на солнце?)
B: Yes, since this morning./Yes, for two hours. (Да, с утра/Да, два часа)

Меди 20%цинка 80%20кг - 80%20%=80%/420кг/4=5кг5кг=20% -> масса меди = 5кг