9. $0.065x$

11. $\$22,000-x$

13.

1. Natasha: ${\displaystyle \frac{1}{4}}t$

2. Natasha’s brother: ${\displaystyle \frac{1}{5}}t$

15.

1. $10-x$

2. $4.60x$

3. $7.40(10-x)$

17.

1. $8+x$

2. $0.8+x$