Tìm x, biết: \(x \times 30\% + x \times \frac{1}{4} + 89 = 100\).

Ta có \(x \times 30\% + x \times \frac{1}{4} + 89 = 100\).

\(x \times \frac{3}{{10}} + x \times \frac{1}{4} = 100 - 89\).

\(x \times \left( {\frac{3}{{10}} + \frac{1}{4}} \right) = 11\).

\(x \times \frac{{11}}{{20}} = 11\).

\(x = 11:\frac{{11}}{{20}}\).

x = 20.

Vậy x = 20.