\frac{s}{t}=1,5 ⇒ \frac{t}{s}= \frac{1}{1,5}= \frac{2}{3} a)- \frac{2t}{3s}=- \frac{2}{3}\cdot \frac{t}{s}= - \frac{2}{3}\cdot \frac{2}{3}=- \frac{4}{9} б)( \frac{s}{t} - 2,5)\cdot \frac{t}{s}= \frac{s}{t} \cdot \frac{t}{s}-2,5\cdot \frac{t}{s} =1- \frac{5}{2} \cdot \frac{2}{3} =1- \frac{5}{3}= \frac{3}{3}- \frac{5}{3}=- \frac{2}{3} в)\frac{2t-3s}{3t} = \frac{2t}{3t} - \frac{3s}{3t} = \frac{2}{3}- \frac{s}{t}= \frac{2}{3}- 1,5= \frac{2}{3}- \frac{3}{2}= \\ \\ = \frac{4}{6}- \frac{9}{6}=- \frac{5}{6}