Explanation

(b)
$k = \frac{0.693}{t_{1/2}}$
$k = \frac{0.693}{10}\ {yr}^{- 1}$
$\ t = \frac{2.303}{k}\log\frac{a}{a - x}$
$\ = \frac{2.303 \times 10}{0.693}\log\frac{a}{a - 0.99a}$
$\ = \frac{23.03}{0.693}\log 100$
$t = \frac{23.03}{0.693} \times 2 = 66.46\ yr \approx 70\ yr$