Ok here's how I got it...

First, find out how the size of the base varies with height. Draw a triangle from the corner of the base to the centre of the base, then up to the peak. This lets you find the angle of the corners, about 56.9 deg. Remember that the distance to the centre is 0.5*sqrt2*the length of side

The length of the side then becomes

(2*sqrt2/tan56.9)*distance from the top of the pyramid

=1.84 * distance from the top. (we'll call this "t")

=1.84 * t

Now find the volume of the pyramid that starts "t" from the peak

V=1/3 * A * t

subbing in our length of side and squaring it to find A

V = 1.133*t^3

Find the mass of the pyramid that's "t" from the peak

M = 200lbs * V

= 226.6 * t^3 lbs*cubic ft

So now we have to add up all the mass that has to be lifted upwards. As we're summing, that's an integration problem

Also, now we have to deal with "height off the ground" as we're lifting. So into our last equation we'll sub "410-r" where r is the height off the ground.

It's a definite integral from 0 to 410.

INT (226.6 * (410-r)^3)dr

I think you have to expand out the cubic term before you integrate and you can take the constant to before the integral.

226.6 * INT (6.89*10^7 - 5*10^5*r + 1230*r^2 - r^3) dr

= 226.6 * (6.89*10^7 *r - 2.5*10^5*t^2 + 410r^3 - (1/4)*t^4) evaluated between 0 and 410

we can just sub in 410 for "r" as when r=0, the integral=0

so when we work it all out we get

Total stone to be lifted = 1.12*10^13 lbs.ft

One worker = 10blocks*50lbs*4ft*10hours*300days*20 years

= 1.2*10^8 lbs.ft

Divide stone by stone per worker = 93,333 workers = THE END

I would strongly recommend going back over all the equations because I rounded heavily in places to avoid having to write too much. But I think that's how you do it. Good luck!