Calculate the total cost of water consumption for 17 kℓ per the tiered pricing structure.

STEP 2 covers water consumption from $> 6 \text{ k}\ell$ up to $10,5 \text{ k}\ell$.

The number of kiloliters in STEP 2 is $\boxed{\text{4,5 k}\ell}$.

• STEP 1: $0 \le 6 \text{ k}\ell$
• STEP 2: $> 6 \le 10,5 \text{ k}\ell$
• STEP 3: $> 10,5 \le 35 \text{ k}\ell$
• STEP 4: $> 35 \text{ k}\ell$ Since $17 \text{ k}\ell$ is greater than $10,5 \text{ k}\ell$ but less than or equal to $35 \text{ k}\ell$, it falls within the range of STEP 3. The last step charged for $17 \text{ k}\ell$ is $\boxed{\text{STEP 3}}$.

• For STEP 1 ($0 \text{ k}\ell$ to $6 \text{ k}\ell$): $$\text{Volume} = 6 \text{ k}\ell$$ $$\text{Rate} = \text{R } 17,51/\text{k}\ell$$ $$\text{Cost}_1 = 6 \text{ k}\ell \times \text{R } 17,51/\text{k}\ell = \text{R } 105,06$$
• For STEP 2 ($> 6 \text{ k}\ell$ to $10,5 \text{ k}\ell$): $$\text{Volume} = 10,5 \text{ k}\ell - 6 \text{ k}\ell = 4,5 \text{ k}\ell$$ $$\text{Rate} = \text{R } 24,39/\text{k}\ell$$ $$\text{Cost}_2 = 4,5 \text{ k}\ell \times \text{R } 24,39/\text{k}\ell = \text{R } 109,755$$
• For STEP 3 ($> 10,5 \text{ k}\ell$ to $17 \text{ k}\ell$): $$\text{Volume} = 17 \text{ k}\ell - 10,5 \text{ k}\ell = 6,5 \text{ k}\ell$$ $$\text{Rate} = \text{R } 34,63/\text{k}\ell$$ $$\text{Cost}_3 = 6,5 \text{ k}\ell \times \text{R } 34,63/\text{k}\ell = \text{R } 225,095$$

The Adams family has to pay $\boxed{\text{R } 439,91}$ for the $17 \text{ k}\ell$ of water used.

The sewerage removal is calculated at a rate of $70\%$ of the total water bill.

Rounding to two decimal places:

They were charged $\boxed{\text{R } 307,94}$ for the sewerage.

Their total water and sewerage bill was $\boxed{\text{R } 747,85}$.