At STP the density of a gas `X` is three times that of gas `Y` while molecule mass of gas Y is twice that of X. The ratio of pressures of X and Y will be:

Explanation

To find the ratio of pressures of gases X and Y, we can use the relationship between density, molecular mass, and pressure. Let's break down the solution step by step.

### Step 1: Understand the given information
We have:
- Density of gas X (Dx) = 3 * Density of gas Y (Dy)
- Molecular mass of gas Y (MY) = 2 * Molecular mass of gas X (MX)

### Step 2: Write the formula for density
The density of a gas can be expressed using the formula:
\[ D = \frac{PM}{RT} \]
Where:
- \( D \) = Density
- \( P \) = Pressure
- \( M \) = Molecular mass
- \( R \) = Universal gas constant
- \( T \) = Temperature

### Step 3: Express the pressures of gases X and Y
From the density formula, we can rearrange it to find pressure:
\[ P = \frac{D \cdot RT}{M} \]

### Step 4: Set up the ratio of pressures
The ratio of pressures of gas X (PX) to gas Y (PY) can be expressed as:
\[ \frac{P_X}{P_Y} = \frac{D_X \cdot R \cdot T / M_X}{D_Y \cdot R \cdot T / M_Y} \]

### Step 5: Simplify the ratio
Since \( R \) and \( T \) are constants and will cancel out, we have:
\[ \frac{P_X}{P_Y} = \frac{D_X}{D_Y} \cdot \frac{M_Y}{M_X} \]

### Step 6: Substitute the known values
Substituting the values we have:
- \( D_X = 3D_Y \)
- \( M_Y = 2M_X \)

So,
\[ \frac{P_X}{P_Y} = \frac{3D_Y}{D_Y} \cdot \frac{2M_X}{M_X} \]

### Step 7: Simplify further
This simplifies to:
\[ \frac{P_X}{P_Y} = 3 \cdot 2 = 6 \]

### Conclusion
Thus, the ratio of pressures of gas X to gas Y is:
\[ \frac{P_X}{P_Y} = 6:1 \]