Question

12+m
$\frac{12 + m}{?20 + m} = 0.90$
What is M?

Answer 1

$\dfrac{12+m}{20+m}=0.9\\\\20+m\neq0\to m\neq-20\\\\0.9=\dfrac{9}{10}\\\\\dfrac{12+m}{20+m}=\dfrac{9}{10}\ \ \ |\text{cross multiply}\\\\(12+m)\cdot10=(20+m)\cdot9\\\\(12)(10)+(m)(10)=(20)(9)+(m)(9)\\\\120+10m=180+9m\ \ \ \ |-120\\\\10m=60+9m\ \ \ |-9m\\\\m=60$