[tex]D= \cfrac{m}{v} \ \ \Rightarrow \ \ m=D\cdot v\\\\ \text{If } \ D= \cfrac{1}{4}
\ \ \text{and } v=m+3 \ \ \text{then:} \\ \\ m= \cfrac{1}{4} \cdot(m+3) \\\\m= \cfrac{1}{4} m+ \cfrac{3}{4} \\ \\ m- \cfrac{1}{4} m= \cfrac{3}{4} \\ \\ \cfrac{3}{4} m= \cfrac{3}{4} \\ \\ \boxed{m=1}[/tex]
