$1-21$ Three blocks $A,B,$ and $C$ and a grooved block $D$ have dimensions $a,b,c,$ and $d$ as

follows:

$a=38\mathrm{.}1+-0\mathrm{.}0254mm,b=50\mathrm{.}800+-0\mathrm{.}0762mm$

$c=76\mathrm{.}2+-0\mathrm{.}1016mm,d=165\mathrm{.}608+-0\mathrm{.}254mm$

$($a$)$ Determine the mean gap $\frac{?}{\text{b}\text{a}\text{r}}(w)$ and its tolerance.

$($b$)$ Determine the mean size of $d$ that will assure that $w0\mathrm{.}254mm.$