7.228. $5^{\lg x}=50-x^{\lg 5}$.
## Solution.

Domain of definition: $0<x \neq 1$.

Rewrite the equation as $5^{\lg x}=50-5^{\lg x}, 2 \cdot 5^{\lg x}=50, 5^{\lg x}=25$, from which $\lg x=2, x=10^{2}=100$.

Answer: 100.