### Theory:

1. If the date of birth of Rahul is $$14/04/2005$$, what is his age on $$04/02/2021$$?

Date of birth $$=$$ $$12/04/2005$$

To find the age on date $$=$$ $$09/02/2021$$ Rahul's age on date $$09/02/2021$$ is $$15$$ years $$9$$ months $$20$$ days.

2. If $$15^{\text{th}}$$ of August $$2020$$ is Saturday, what is the day on $$10^{\text{th}}$$ January $$2021$$.

Solution:

Given: $$15^{\text{th}}$$ of August $$2020$$ is Saturday.

Number of days in August $$=$$ $$31-14 =17$$

Number of days in September $$=$$ $$30$$

Number of days in October $$=$$ $$31$$

Number of days in November $$=$$ $$30$$

Number of days in December $$=$$ $$31$$

Number of days in January $$=$$ $$10- 1 = 9$$

Total number of days $$=$$ $$17 + 30 + 31 + 30 + 31 + 9 = 148$$

A week has $$7$$ days.

So, divide $$148$$ days by $$7$$ days.

$\begin{array}{l}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}21\\ 7\overline{)148}\\ \underset{¯}{\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}14\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}}\\ \phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}8\\ \underset{¯}{\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}7\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}}\\ \underset{¯}{\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}1\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}}\phantom{\rule{0.147em}{0ex}}\end{array}$

$$148$$ days $$=$$ $$21$$ weeks $$+$$ $$1$$ day

So, the required day is $$1$$ day after Saturday.

Therefore, $$10^{\text{th}}$$ January $$2021$$ is Sunday.

Important!
If the remainder is:

$$0$$ - Same given day

$$1$$ - One day after the given day

$$2$$ - Two days after the given day

$$3$$ - Three days after the given day

$$4$$ - Four days after the given day

$$5$$ - Five days after the given day

$$6$$ - Six days after the given day