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Chemistry Level 3

$\color{limegreen}{\underline{\text{Chemistry Daily Challenge }}}$ $\color{#20A900}{\text{29-July-2015}}$

Three elements A, B, and C form three binary compounds. Each element has the same valence in these compounds. The mass fraction of A in the compound with B is 75%, and the mass fraction of B in the compound with C is 7.8%. Determine the percentage of C in the compound with A.