69.

1. $C(K)=K-273;$ it represents the Celsius temperature corresponding to a given Kelvin temperature.

2. 27°C

3. 295 K

71.

1. $F(K(C))={\displaystyle \frac{9}{5}}C+32$

2. $C(K(F))={\displaystyle \frac{5}{9}}F={\displaystyle \frac{160}{9}}$

73.

1. Dollars to euros: $f(x)=0.75x$ (x is the number of dollars; f(x) is the number of euros); euros to dollars: $g(x)=1.25x$ (x is the number of euros; g(x) is the number of dollars)

2. She loses money.

75.

1. $w=\{\begin{array}{ll}4+0.05x,& \text{if}x>60\\ 7,& \text{if}x\le 60\end{array}$

2. It does not have an inverse because it is constant on (0, 60) and hence is not one-to-one.

3. Restriction of the domain: $[60,\infty )$

77.

1. $x={\displaystyle \frac{1}{64}}{V}^2;$ it gives the height of the water in terms of the velocity.

2. 1. 14.0625 ft

   2. 6.25 ft

79.

1. The balance due after x months

2. $f^{-1}(x)=60-{\displaystyle \frac{1}{600}}x;$ it shows the number of months that have passed from the first payment until the balance due is $x

3. $23.333\approx 24\text{ months}$