G6.1 If $p=\frac{21^{3}-11^{3}}{21^{2}+21 \times 11+11^{2}}$, find $p$. (Similar questions: 1985 FG7.1)
G6.2 If $p$ men can do a job in 6 days and 4 men can do the same job in $q$ days, find $q$.
G6.3 If the $q^{\text {th }}$ day of March in a year is Wednesday and the $r^{\text {th }}$ day of March in the same year is
Friday, where $18<r<26$, find $r$. (Reference: 1985 FG9.3, 1987 FG6.4, 1988 FG10.2)

G6.4 If $a * b=a b+1$, and $s=(3 * 4)^{*}$, find $s$. (Reference: 1985 FSG.1)
$$
p=\frac{21^{3}-11^{3}}{21^{2}+21 \times 11+11^{2}}=\frac{(21-11)\left(21^{2}+21 \times 11+11^{2}\right)}{21^{2}+21 \times 11+11^{2}}=10
$$
10 men can do a job in 6 days.
1 man can do a job in 60 days
4 men can do a job in 15 days $\Rightarrow q=15$
$15^{\text {th }}$ March is Wednesday
$17^{\text {th }}$ March is Friday
$24^{\text {th }}$ March is Friday $\Rightarrow r=24$
$$
\begin{array}{l}
3 * 4=3 \times 4+1=13 \\
s=(3 * 4) * 2=13 * 2=13 \times 2+1=27
\end{array}
$$