A heavy nucleus Q of half-life 20 minutes undergoes alpha-decay...

Given,

$T\dfrac{1}{2} = 20min$

${N}_{0} = 1000$

$∵ 1hour = 60min = 3T\dfrac{1}{2}$

$∴$ Number of nuclei remain after $1$ hour,

$N = \dfrac{{N}_{o}}{{2}^{3}} = \dfrac{1000}{8}$

$⇒ N = 125$

It means $875$ nuclei has decayed,

Now,

Number of $\alpha$ decayed Nucleus

$= \dfrac{875 \times 60}{100} = 525$