Please give them in writing

The ratio 1:2:4 means 1 part cement, 2 parts sand, and 4 parts aggregate.
Given that 2 parts of sand correspond to 18 buckets:

$$2 \text{ parts of sand} = 18 \text{ buckets}$$ $$1 \text{ part} = \frac{18 \text{ buckets}}{2} = 9 \text{ buckets}$$

Cement is 1 part:

$$\text{Cement} = 1 \times 9 \text{ buckets} = 9 \text{ buckets}$$

Aggregate is 4 parts:

$$\text{Aggregate} = 4 \times 9 \text{ buckets} = 36 \text{ buckets}$$

The quantity of cement needed is $\boxed{\text{9 buckets}}$ and the quantity of aggregate needed is $\boxed{\text{36 buckets}}$.

Given:
Length ($L$) = $6 \text{ m}$
Width ($W$) = $3 \text{ m}$
Thickness ($T$) = $150 \text{ mm}$
To convert millimeters to meters, divide by 1000:

$$T = \frac{150 \text{ mm}}{1000 \text{ mm/m}} = 0.15 \text{ m}$$

The volume ($V$) of a slab is calculated as:

$$V = L \times W \times T$$

$$V = 6 \text{ m} \times 3 \text{ m} \times 0.15 \text{ m}$$ $$V = 18 \text{ m}^2 \times 0.15 \text{ m}$$ $$V = 2.7 \text{ m}^3$$

The volume of concrete needed is $\boxed{\text{2.7 m}^3}$.