I'll use this CW answer to gather together some deductions from the
power series expansion
$$
\frac12\log\frac{1+x}{1-x} = x+\frac{x^3}3+\frac{x^5}5+\cdots.
$$
This can be truncated to give lower bounds, and its remainder can be
replaced by a geometric series with common ratio $x^2$ to give upper
bounds.
Taking the first question first:
\begin{multline*}
50\log\frac{21}{20} =
100\cdot\frac12\log\frac{1+\tfrac1{41}}{1-\tfrac1{41}} <
100\cdot\frac1{41}\left(1-\frac1{41^2}\right)^{-1} \\ =
\frac{100\cdot41}{41^2-1} = \frac{100\cdot41}{40\cdot42} =
\frac{205}{84} < \frac52,
\end{multline*}
whence
$$
\left(\frac{21}{20}\right)^{50} < e^{5/2} < 3^{5/2} = \sqrt{243} <
\sqrt{256} = 16 < 100. \quad \square
$$
It doesn't take much more work to get a sharper upper bound:
$$
\left(\frac{21}{20}\right)^{50} < 12.
$$
Proof:
$$
50\log\frac{21}{20} < \frac{205}{84} < \frac{49}{20} < \log{12}.
$$
The penultimate inequality follows from
$$
20\cdot205 = 4100 < 4116 = 4200 - 84 = 50\cdot84 - 84 = 49\cdot84.
$$
The final inequality is proved as follows:
\begin{gather*}
\log3 = 2\cdot\frac12\log\frac{1+\tfrac12}{1-\tfrac12} >
2\left(\frac12 + \frac1{3\cdot2^3}\right) = \frac{13}{12}; \\
\log2 = 2\cdot\frac12\log\frac{1+\tfrac13}{1-\tfrac13} >
2\left(\frac13 + \frac1{3^4}\right) = \frac{56}{81}; \\
\log{12} = \log3 + 2\log2 > \frac{13}{12} + \frac{112}{81} =
\frac{351+448}{324} = \frac{799}{324} > \frac{800}{325} =
\frac{32}{13} > \frac{49}{20},
\end{gather*}
because $20\cdot32 = 640 > 637 = 650-13 = 50\cdot13-13 = 49\cdot13.
\quad \square$
An answer to the OP's second question can be deduced from the
inequality in the first question. As @Federico notes in his answer,
the second question reduces to
$$
\left(\frac{2^{10}}{10^3}\right)^{100} < 100.
$$
Therefore, it is enough to prove
$$
\left(\frac{2^{10}}{10^3}\right)^2 = \left(1 + \frac3{125}\right)^2
< 1 + \frac1{20} = \frac{21}{20};
$$
and the central inequality does hold, because
$$
\frac1{20} - \left(\frac6{125} + \frac9{125^2}\right) =
\left(\frac6{120} - \frac6{125}\right) - \frac9{125^2} =
\frac1{125}\left(\frac14 - \frac9{125}\right) > 0.
\quad \square
$$
(With hindsight, it would have been simpler just to write
$1.024^2 = 1.048576 < 1.05$!)
Alternatively, we can answer the second question directly, using the
same power series again:
\begin{multline*}
100\log\frac{2^{10}}{10^3} = 100\log\left(1+\frac6{250}\right) =
200\cdot\frac12\log\frac{1+\tfrac3{253}}{1-\tfrac3{253}} \\
< \frac{600}{253}\left(1-\left(\frac3{253}\right)^2\right)^{-1}\!\!
= \frac{600\cdot253}{253^2-3^2} = \frac{600\cdot253}{250\cdot256} =
\frac{3\cdot253}{5\cdot64} = \frac{759}{320} < \frac52,
\end{multline*}
whence, exactly as with the first question,
$$
\left(\frac{2^{10}}{10^3}\right)^{100} < e^{5/2} < 3^{5/2} =
\sqrt{243} < \sqrt{256} = 16. \quad \square
$$
Also in his answer, @Federico poses (and solves) the problem of
proving
$$
\left(\frac{2^{10}}{10^3}\right)^{100} > 10.
$$
We can prove this by using the power series again. To begin with,
$$
100\log\frac{2^{10}}{10^3} = 100\log\left(1+\frac6{250}\right) =
200\cdot\frac12\log\frac{1+\tfrac3{253}}{1-\tfrac3{253}} >
\frac{600}{253} > \frac73.
$$
@Federico gives a short and clear proof that $e^{7/3} > 10$ by using
the power series for $e^x,$ and this is probably to be preferred;
but I'll give another proof, using the same power series I've been
using throughout. (There are several other reasonable-looking ways
to use the same power series to prove the result, and I make no claim
that this is the neatest.)
$$
\log3 = 2\cdot\frac12\log\frac{1+\tfrac12}{1-\tfrac12} <
2\left(\frac12 + \frac1{3\cdot2^3}\left(
1-\frac1{2^2}\right)^{-1}\right) = 2\left(\frac12+\frac1{18}\right)
= \frac{10}9;
$$
\begin{multline*}
\log\frac{10}9 = 2\log\frac{1+\tfrac1{19}}{1-\tfrac1{19}} <
2\left(\frac1{19} + \frac1{3\cdot{19}^3}\left(
1-\frac1{{19}^2}\right)^{-1}\right) = \\
2\left(\frac1{19} + \frac1{3\cdot19\cdot(19^2-1)}\right) =
2\left(\frac1{19} + \frac1{3\cdot18\cdot19\cdot20}\right) <
2\left(\frac1{19} + \frac1{18\cdot19}\right) = \frac19;
\end{multline*}
$$
\log{10} = 2\log3 + \log\frac{10}9 < \frac{20}9 + \frac19 = \frac73.
\quad \square
$$
That will have to be enough for today, but it would be fun to give
similar proofs of the two middle inequalities here:
$$
10 < \left(\frac{2^{10}}{10^3}\right)^{100}\!\! < 11 <
\left(\frac{21}{20}\right)^{50}\! < 12,
$$
and as this is a CW answer, anyone should feel free to append
such proofs.