Use the definition of conditional probability to find the "and" probability:
[tex]P(Y\mid X)=\dfrac{P(X\cap Y)}{P(X)}\implies P(X\cap Y)=0.3\times0.5=0.15[/tex]
Use the inclusion/exclusion principle to find the "or" probability:
[tex]P(X\cup Y)=P(X)+P(Y)-P(X\cap Y)=0.5+0.4-0.15=0.75[/tex]
