Chapter 13 - Counting and Probability - 13.3 Probability - 13.3 Asses Your Understanding - Page 866: 66

$0.6$

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 $n(E)=30+20+10=60$ houses with an income at least $20000\$$, in total we have $n(S)=100$ houses, therefore $\text{probability}=\frac{60}{100}=0.6.$