[tex]\bf \underset{10x}{L\stackrel{7x+4}{\rule[0.35em]{10em}{0.25pt}}M\stackrel{20}{\rule[0.35em]{15em}{0.25pt}}N}
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LN=LM+MN\implies \stackrel{LN}{10x}=\stackrel{LM}{(7x+4)}+\stackrel{MN}{20}\implies 10x=7x+24
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3x=24\implies x=\cfrac{24}{3}\implies \boxed{x=8}
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LM=7(8)+4\implies LM=60[/tex]
