(20-9-7)a/15+(9-10-1)b/6
4a/15-b/3
(4a-5b)/15
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$\small \left(\dfrac{4}{3}a-\dfrac{1}{6}b\right)-\left(\dfrac{3}{5}a+\dfrac{5}{3}b\right)+\left(-\dfrac{7}{15}a+\dfrac{3}{2}b\right)=$
$\small =\dfrac{4}{3}a-\dfrac{1}{6}b-\dfrac{3}{5}a-\dfrac{5}{3}b-\dfrac{7}{15}a+\dfrac{3}{2}b =$
$\small =\left(\dfrac{4}{3}-\dfrac{3}{5}-\dfrac{7}{15}\right)a+\left(-\dfrac{1}{6}-\dfrac{5}{3}+\dfrac{3}{2}\right)b =$
$\small =\left(\dfrac{20-9-7}{15}\right)a+\left(\dfrac{-1-10+9}{6}\right)b =$
$\small = \dfrac{4}{15}a+\left(-\dfrac{\cancel2^1}{\cancel6_3}\right)b =$
$\small = \dfrac{4}{15}a-\dfrac{1}{3}b $