A) Calculate the total water used in 3 days if Day 1 used sqrt(5)L, Day 2 used 2sqrt(5)L, and Day 3 used 3sqrt(5)L. b) Calculate the total cost if water costs Ksh 15 per litre.

Day 1: $\sqrt{5}$ litres
Day 2: $2\sqrt{5}$ litres
Day 3: $3\sqrt{5}$ litres

Total water $= \sqrt{5} + 2\sqrt{5} + 3\sqrt{5}$
Total water $= (1 + 2 + 3)\sqrt{5}$
Total water $= 6\sqrt{5}$ litres
The total water used is $\boxed{6\sqrt{5} \text{ litres}}$.

b)

Cost per litre = Ksh 15
Total water used = $6\sqrt{5}$ litres

Total cost $= \text{Total water used} \times \text{Cost per litre}$
Total cost $= 6\sqrt{5} \times 15$
Total cost $= 90\sqrt{5}$ Ksh
The total cost is $\boxed{90\sqrt{5} \text{ Ksh}}$.

17. a)

Battery life of one device $= \sqrt{11}$ hours

Total time for 8 phones $= 8 \times \sqrt{11}$
Total time for 8 phones $= 8\sqrt{11}$ hours
The total time for 8 phones is $\boxed{8\sqrt{11} \text{ hours}}$.

b)

Expression $= \frac{4}{\sqrt{11}}$

The rationalized expression is $\boxed*{\frac{4\sqrt{11}}{11}*}$.

18. a)

Expression $= 2x^2 - 13x + 20$

The numbers are $-5$ and $-8$.

The factorized expression is $\boxed{(2x - 5)(x - 4)}$.

b)

The solutions are $\boxed*{x = \frac{5}{2} \text{ and } x = 4*}$.

19. a)

The simplified expression is $\boxed{ab^2}$.

b)

The numerical value is $\boxed{18}$.

20. a)

Find two numbers that multiply to $-21$ and add to $-4$.
The numbers are $-7$ and 3.

The dimensions of the garden are $\boxed{(x - 7) \text{ and } (x + 3) \text{ units}}$. (Assuming units are in metres, for example).

b)

The dimensions are $(x - 7)$ and $(x + 3)$.
Perimeter $P = 2(\text{length} + \text{width})$

Cost per metre = KSh 150
Total cost $= \text{Perimeter} \times \text{Cost per metre}$

The cost of fencing the garden is $\boxed{(600x - 1200) \text{ Ksh}}$.