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Ответ:

Это 4

[tex]x \times 1 \frac{3}{4} = 7 \\ [/tex]

[tex]x = 7 \div \frac{7}{4} = 7 \times \frac{4}{7} [/tex]

[tex]x = \frac{4}{1} [/tex]

Проверяем....

[tex]4 \times \frac{7}{4} = 7[/tex]

[tex]4 \times \frac{7}{4} = \frac{7}{1} \\ \frac{7}{1} = 7 \\ 7 = 7[/tex]