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?? ;-)
This is a linear equation or y=mx+c. Where y is the number of mosquitos at a particular month and x is the number of months. We know the initial population of the mosquitoes is c=20. They population doubles every month so this is the gradient, m=2. Therefore the equation for the growth of the mosquito population is:
y = 2x + 20.
So after x= 10 months the mosquito population will be,
y=2(10)+20= 40.
There will be 40 mosquitoes after ten months.
For 4x^2y over 2x^8y^4 try 2 over x^6y^3
