2/3+2/3=4/3

lượng muối trong 400 gam nước biển là:

400x4:100=16 ( g)

16 gam cần số nước là: 

100:2x16=800 ( g)

cần thêm là: 

800-400=400( g)

đáp số: 400 gam

2,07+ 4,15 +17,93 +5,85= 30

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