Explanation

(b)
In a leap year there are 366 days in which 52 weeks and two days. The combination of 2 days may be: Sun-Mon, Mon-Tue, Tue-Wed, Wed-Thu, Thu-Fri, Fri-Sat, Sat-Sun.
$P(53Fri) = \frac{2}{7};P(53\ Sat) = \frac{2}{7}$ and $P(53\ Fri\ and\ 53\ Sat) = \frac{1}{7}$
$\therefore P(53\ Fri\ or\ \ Sat) = P(53\ Fri) + P(53\ Sat) - P(53\ Fri\ and\ Sat)$
$= \frac{2}{7} + \frac{2}{7} - \frac{1}{7} = \frac{3}{7}$