Okay! Some nice explanations there. Here's my take...
Total Houses: 100.
Houses with DVD player: 75
Houses w/ Cell Phones: 80
Houses w/ MP3s: 55
That's all the data we have. Now we need to find the max and min values of 'g' (remember, that represents the 'all-the-three' block in the Venn Diagram).
Lets focus on the max value of 'g':
If we go on distributing a set of those three gadgets to each house, we'll run out of MP3s after House No. 55. So, we'll stop right there. That's the max number of houses which can have all the three gadgets. (g max = 55)
Lets now focus on the min value of 'g':
We don't have to begin from the start. Let's continue from where we left in our previous step.
55 houses have all the three gadgets. Let's call it Block A. 45 houses are still empty, and this is Block B. We have 80-55 = 25 CD players, and 75-55=20 DVD players left for distribution.
Total gadgets left: 45. Total houses left: 45. Voila! Give each one away to each house left.
Now, let's minimise the no. of houses having all the three gadgets. How can we do that?
None of the houses in Block B have an MP3 player. Okay, lets remove 45 MP3s from the Block A and re-distribute it to Block B.
55-45=10. Just 10 houses now have all the three gadgets.
Focus on the second step, since that is the trickiest part of this solution. Remember, you can't remove any more DVDs or CDs from Block A, since all the houses in Block B have them already. So, you can only remove the MP3s from Block A.
There you go: 55-10 = 45!
Cheers!