For the rectangular footing shown in Fig. P$7\mathrm{.}20,$ sketch the problem and calculate the vertical stress increase $({}^{\text{'}})$ below points A and B only, at $6m$ and $15m$ of depth. Use the chart given in class or figure $7\mathrm{.}24$ of the textbook. Note that point $A$ is below the center and not below a corner.

$7\mathrm{.}20$ A rectangular foundation $4m6m($Figure P$7\mathrm{.}20)$ transmits a stress of $100$kPa on the surface of a soil deposit. Plot the distribution of increases of vertical stresses with depth under points $A,B,$ and $C$ up to a depth of $20m.$ At what depth is the increase in vertical stress below A less than $10\%$ of the surface stress?