Answer: E

Small Hint:

Near the relevant slopes, the number of points is $\sum_{x=1}^{30}\lfloor mx\rfloor$

Big Hint:

Find the slope where the $300$th lattice-point ratio occurs and the next larger possible ratio

Solution:

For a fixed slope $m,$ the number of points in $S$ on or below $y=mx$ is

$$\sum_{x=1}^{30}\lfloor mx\rfloor,$$

for the slopes near the answer.

At $m=\frac23,$ grouping $x=3k+1,3k+2,3k+3$ for $k=0,1,\ldots,9$ gives

$$\lfloor \frac{2x}{3}\rfloor=2k,\ 2k+1, \ 2k+2,$$

whose sum over each block is $6k+3.$ Thus the total is

$$\sum_{k=0}^9(6k+3)=270+30=300.$$

If $m<\frac23,$ the ten points with ratios $\frac{y}{x}=\frac{2}{3}$ are no longer counted, so the count is less than $300.$ Therefore the lower end is $\frac23.$

The next possible ratio $\frac{y}{x}$ greater than $\frac23,$ with $1\le x,y\le30,$ is minimized by checking $x$ modulo $3.$ The best candidates are

$$\frac{19}{28},\qquad \frac{20}{29},\qquad \frac{21}{30}=\frac{7}{10},$$

and the smallest is $\frac{19}{28}.$ Hence the interval length is

$$\frac{19}{28}-\frac23=\frac1{84}.$$

Thus $a+b=1+84=85.$

Thus, the answer is E .