[tex]\dfrac{12+m}{20+m}=0.75\\\\20+m\neq0\to m\neq-20\\\\0.75=\dfrac{3}{4}\\\\\dfrac{12+m}{20+m}=\dfrac{3}{4}\ \ \ |\text{cross multiply}\\\\(12+m)\cdot4=(20+m)\cdot3\\\\(12)(4)+(m)(4)=(20)(3)+(m)(3)\\\\48+4m=60+3m\ \ \ |-48\\\\4m=12+3m\ \ \ |-3m\\\\m=12[/tex]
