Răspuns :
[tex] \frac{X}{7} = \frac{15}{21} ~SIMPLIFICAM~PRIN~3: \frac{x}{7} = \frac{5}{7} \\\\
DIN~PROPORTIA~NOU~OBTINUTA~AFLAM~X-UL\\
X=(7*5):7~SIMPLIFICAM~PRIN~7~SI~AVEM:X=5\\\\
30= \frac{15}{X} =\ \textgreater \ X=15:30~SIMPLIFICAM~PRIN~15\\\\
SI~OBTINEM~CA~X= \frac{1}{2} [/tex]