(iii) Total number of black balls is $5$
We know that the Probability = Number of favourable outcomes/ Total number of outcomes
Therefore, the probability of drawing black ball P(E)$=5/15=1/3$
But, this is not what is required.
As we know that sum of probability of occurrence of an event and probability of non-occurrence of an event is $1$
$P(E)+P(\overset{\scriptscriptstyle\rightharpoonup}{E})=1$
$\frac{1}{3}+P(\overset{\scriptscriptstyle\rightharpoonup}{E})=1$
$P(\overset{\scriptscriptstyle\rightharpoonup}{E})=1-\frac{1}{3}$
$P(\overset{\scriptscriptstyle\rightharpoonup}{E})=\frac{2}{3}$
Hence, the probability of drawing a card that is not black is $2/3$