[tex]the\ product\ of\ 4\ an\ number\ x\to4\cdot x=4x\\\\4x \ \textless \ 21\ \ \ \ |divide\ both\ sides\ by\ 4\\\\x \ \textless \ \dfrac{21}{4}\to x \ \textless \ \dfrac{20+1}{4}\to x \ \textless \ \dfrac{20}{4}+\dfrac{1}{4}\to x \ \textless \ 5+\dfrac{1}{4}\\\\\huge\boxed{x \ \textless \ 5\frac{1}{4}\to x\in\left(-\infty;\ 5\frac{1}{4}\right)}[/tex]
