Fill in the blanks: a) 25.12 x 10 = _ b) x 100 = 546.2 c) x 1000 = 508.2 d) 0.0045 x 10,000 = _

The blank is $\boxed{251.2}$.

b)

The equation is $x \times 100 = 546.2$.

The blank is $\boxed{5.462}$.

c)

The equation is $y \times 1000 = 508.2$.

The blank is $\boxed{0.5082}$.

d)

The blank is $\boxed{45}$.

2. Multiply:

a)

The product is $\boxed{3249.6}$.

b)

The product is $\boxed{819.54}$.

c)

The product is $\boxed{262.15}$.

d)

The product is $\boxed{8.64}$.

3. Multiply:

a)

The product is $\boxed{1510.92}$.

b)

The product is $\boxed{4265.752}$.

c)

The product is $\boxed{513.4094}$.

d)

The product is $\boxed{32.53}$.

4.

Weight of one coin $= 5.25 \text{ grams}$
Number of coins $= 15$
Total weight $= 5.25 \text{ g} \times 15$

15 such coins will weigh $\boxed{78.75 \text{ grams}}$.

5.

Cost of $1 \text{ kg}$ biscuits $= ₹162$
Quantity bought $= 1.25 \text{ kg}$
Total cost $= ₹162 \times 1.25$

Charles spent $\boxed{₹202.50}$ on biscuits.
A healthier snack item is fruit (e.g., an apple or banana).

6.

Cost per metre of hose pipe $= ₹35.50$
Length of hose pipe needed $= 8.35 \text{ metres}$
Total cost $= ₹35.50 \times 8.35$

The cost of 8.35 metres length of hose pipe is $\boxed{₹296.925}$.
It is important to save water because it is a finite natural resource essential for all life, and conserving it helps maintain ecosystems and ensures availability for future generations.