Let's first break it down to the time 8 people need for one wall. That would be 10 minutes, right? If one person would have to do it in stead of 8, you'd expect him to take 80 minutes.
To paint 7 walls, he would nee 7x80 = 560 minutes. 10 people would do that 10 times faster, ie., 56 minutes.
I know puzzles like these always have a mindtrick. Did I miss anything?? ;-)
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Hello! remember you have to write complete questions in order to get good and exact answers. Here I'll explain this in a general way assuming the following table:

As you can see, x increases in steps of 1 units. So let's check the difference in y:

So we have a constant difference and we can conclude the table represents a line. Since the line passes through then y is directly proportional to x, so we can write:

Then:

Finally, the equation of the line is:

Let, that number = x
It would be: x * 0.65 = 52
x = 52 / 0.65
x = 80
So, that number and your answer is 80
Hope this helps!