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`a)` `-` `C% =` $\dfrac{m_{ct}}{m_{dd}}$ ` × 100% `
`->` $m_{dd(NaCl)}$ `=` `\frac{m_{ct}}{C%} × 100% =` $\dfrac{50}{25}$ `× 100 = 200` `(g)`
`b)` `-` $m_{dd(NaCl)}$ `=` $m_{ct}$ `+` $m_{nước}$
`->` $m_{nước}$ `=` $m_{dd(NaCl)}$ `-` $m_{ct}$ `= 200 - 50 = 150` `(g)`