3.
[tex]M = \frac{ab}{c} | \times c \\ Mc = ab | \div a \\ b = \frac{Mc}{a} [/tex]
4.
a)
[tex]4dol. - 3euro \\ 20dol. - x \\ \\ x = \frac{20 \times 3}{4} = 5 \times 3 = \boxed{15euro}[/tex]
b)
[tex]200km - 2h \\ 325km - x \\ \\ x = \frac{325 \times 2}{200} = \frac{325}{100} = \boxed{3.25h}[/tex]
