#### Answer

$0.3$

#### Work Step by Step

We know that $\text{probability}=\frac{\text{number of favourable outcomes}}{\text{number of all outcomes}}=\frac{n(E)}{n(S)}.$
Hence here because we have $n(E)=20+10=30$ houses with at least $30000\$$ income, in total we have $n(S)=100$ houses, therefore $\text{probability}=\frac{30}{100}=0.3.$