To keep your arrangement of 20 white tulips in a glass vase fresh and beautiful, follow these care instructions:

1. **Fresh Cut**: Upon receiving the tulips, trim about an inch off the bottom of each stem at a sharp angle. This will help the tulips absorb water more effectively.

2. **Clean Vase**: Fill a clean vase with lukewarm water and add any flower preservatives if provided. Make sure the vase is thoroughly clean to prevent bacterial growth.

3. **Water Level**: Place the tulips in the vase and ensure that the stems are submerged in water. Check the water level daily and top up as needed to keep the stems hydrated.

4. **Location**: Display the vase in a cool location away from direct sunlight, heat sources, and drafts. Tulips are sensitive to heat and will wilt quickly if exposed to high temperatures.

5. **Changing Water**: Replace the water in the vase every two days or when it becomes cloudy. Before adding fresh water, re-trim the stems to maintain their ability to absorb water.

6. **Remove Foliage**: Remove any leaves that will be submerged in water to prevent them from decaying and contaminating the water.

7. **Ethylene Sensitivity**: Keep the tulips away from fruits and vegetables, as they produce ethylene gas, which can cause the flowers to deteriorate more quickly.

8. **Enjoyment**: Finally, enjoy the beauty of your arrangement! Remove any wilted flowers promptly to maintain the overall appearance of the display.

By following these care tips, you can extend the lifespan of your tulip arrangement and enjoy its beauty for as long as possible.