Definitions:
Completed rule
\(\displaystyle{P}{\left({A}^{{c}}\right)}={P}{\left(\neg{A}\right)}={1}-{P}{\left({A}\right)}\)
General addition rule for any two events:
PSKP(A or B) = P(A) + P(B) - P(A and B)
Solution
\(\displaystyle{b}{e}{g}\in{\left\lbrace{a}{r}{r}{a}{y}\right\rbrace}{\left\lbrace{l}{\left|{c}\right|}{c}{\left|{c}\right|}{c}\right\rbrace}&{b}{e}{g}\in{\left\lbrace{a}{r}{r}{a}{y}\right\rbrace}{\left\lbrace{c}\right\rbrace}{4}{m}{a}{t}{h}{r}{m}{\left\lbrace{t}{h}\right\rbrace}\ \nabla{e}{e}{n}{d}{\left\lbrace{a}{r}{r}{a}{y}\right\rbrace}&{b}{e}{g}\in{\left\lbrace{a}{r}{r}{a}{y}\right\rbrace}{\left\lbrace{c}\right\rbrace}{5}{m}{a}{t}{h}{r}{m}{\left\lbrace{t}{h}\right\rbrace}\ \nabla{e}{e}{n}{d}{\left\lbrace{a}{r}{r}{a}{y}\right\rbrace}&{b}{e}{g}\in{\left\lbrace{a}{r}{r}{a}{y}\right\rbrace}{\left\lbrace{c}\right\rbrace}{6}{m}{a}{t}{h}{r}{m}{\left\lbrace{t}{h}\right\rbrace}\ \nabla{e}{e}{n}{d}{\left\lbrace{a}{r}{r}{a}{y}\right\rbrace}&{T}{o}{t}{a}{l}\backslash{h}{l}\in{e}{G}{r}{a}{d}{e}{s}&{49}&{50}&{69}&{168}\backslash{h}{l}\in{e}{A}{t}{h}\le{t}{i}{c}&{24}&{36}&{38}&{98}\backslash{h}{l}\in{e}{P}{o}{p}\underline{{a}}{r}&{19}&{22}&{28}&{69}\backslash{h}{l}\in{e}{T}{o}{t}{a}{l}&{92}&{108}&{135}&{335}{e}{n}{d}{\left\lbrace{a}{r}{r}{a}{y}\right\rbrace}\)
S = Sixth grader
G = Grades
We note that 135 of the 335 people in the table are 6th grades, because 135 is mentioned in
the row ” Total” and in the column ”6th grade” of the given table.
The probability is the number of favorable outcomes divided by the number
of possible outcomes:
\(\displaystyle{P}{\left({S}\right)}={\frac{{#{o}{f}{f}{a}{v}{\quad\text{or}\quad}{a}{b}\le{o}{u}{t}{c}{o}{m}{e}{s}}}{{#{o}{f}{p}{o}{s}{s}{i}{b}\le{o}{u}{t}{c}{o}{m}{e}{s}}}}={\frac{{{135}}}{{{335}}}}\)
We note that 168 of the 335 people in the table rated good grades as important, because 168 is mentioned in the row ” Grades” and in the column ”Total” of the given table.
\(\displaystyle{P}{\left({G}\right)}={\frac{{#{o}{f}{f}{a}{v}{\quad\text{or}\quad}{a}{b}\le{o}{u}{t}{c}{o}{m}{e}{s}}}{{#{o}{f}{p}{o}{s}{s}{i}{b}\le{o}{u}{t}{c}{o}{m}{e}{s}}}}={\frac{{{168}}}{{{335}}}}\)
We note that 69 of the 335 people in the table are 6th graders who rated good grades as important, because 69 is mentioned in the row ” Grades” and in the column ”6th gradel” of the given table.
\(\displaystyle{P}{\left({G}\right)}={\frac{{#{o}{f}{f}{a}{v}{\quad\text{or}\quad}{a}{b}\le{o}{u}{t}{c}{o}{m}{e}{s}}}{{#{o}{f}{p}{o}{s}{s}{i}{b}\le{o}{u}{t}{c}{o}{m}{e}{s}}}}={\frac{{{69}}}{{{335}}}}\)
Use the general addition rule:
\(\displaystyle{P}{\left({S}{\quad\text{or}\quad}{G}\right)}={P}{\left({S}\right)}+{P}{\left({G}\right)}-{P}{\left({S}{\quad\text{and}\quad}{G}\right)}\)

\(\displaystyle={\frac{{{135}}}{{{335}}}}+{\frac{{{168}}}{{{335}}}}-{\frac{{{69}}}{{{335}}}}\)

\(\displaystyle={\frac{{{135}+{168}-{69}}}{{{335}}}}\)

\(\displaystyle={\frac{{{234}}}{{{335}}}}\)

\(\displaystyle\approx{0.6985}\)

\(\displaystyle={69.85}\%\)

\(\displaystyle={\frac{{{135}}}{{{335}}}}+{\frac{{{168}}}{{{335}}}}-{\frac{{{69}}}{{{335}}}}\)

\(\displaystyle={\frac{{{135}+{168}-{69}}}{{{335}}}}\)

\(\displaystyle={\frac{{{234}}}{{{335}}}}\)

\(\displaystyle\approx{0.6985}\)

\(\displaystyle={69.85}\%\)