Physics: Reasonable Estimates Of Quantities (Random Questions)

Well, in A-Level physics, you'll often be asked to make reasonable estimate of physical quantities. I think it's a really mind triggering question. Here are some of the questions along with their answers that i found in Paper 2. I'll update the list if i find more questions like these.

1. Frequency Of Audible Sound Wave: 
=> Human ear cannot detect sound which has frequency less than 20Hz or Greater than 20KHz. So, Frequency of Audible Sound Wave must be greater than 20Hz and less than 20KHz. Answer: 5KHz.

2. The Wavelength, in nm, of Ultraviolet Radiation:
=> If you remember the Order Of Magnitude of electromagnetic radiation then you can answer this question easily. Ultraviolet radiation has wavelength from meter. So you can put any value between this. Answer:  50nm

3. The Mass Of The Plastic 30cm Ruler:
=> A plastic ruler cannot have mass greater or equal to KG. But a 500 gram object will obviously be much heavier than a plastic ruler. So, it must be in the range of 30g to 100g. Answer: 50g

4. Density Of Air At Atmospheric
=> As we know the value of Atmospheric pressure if approximately 100,000 pascals. Let's consider the height of the air that surrounds the earth is 10000m. Now using the formula Pressure = density * gravity * height, we can find out the density. Density: 1 Kg/m^3