5z+15z=840
20z=840
z=840/20
z=42
проверка
5*42+15*42=840
210+630=840
840=840
1)5z+15z=840, 20z=840, z=840:20,z=42, проверка: 5*42+15*42=840, 210+630=840, 840=840.
5z+15z=840
20z=840
z=840/20
z=42
проверка
5*42+15*42=840
210+630=840
840=840
1)5z+15z=840, 20z=840, z=840:20,z=42, проверка: 5*42+15*42=840, 210+630=840, 840=840.