For the subtraction of two unlike fractions, first, change them to like fractions.

LCM of 10, 15 = 30

Now, let us change each of the given fractions into an equivalent fraction having 30 as the denominator.

Therefore,

$\frac{9}{10}=\frac{9\times 3}{10\times 3}=\frac{27}{30}$

$\frac{4}{15}=\frac{4\times 2}{15\times 2}=\frac{8}{30}$

Now,

$\frac{9}{10}-\frac{4}{15}$

$=\frac{27}{30}-\frac{8}{30}$

$=\frac{27-8}{30}$

$=\frac{\mathbf{19}}{\mathbf{30}}$