Part (a)
For 1 plank

$I_1 = \dfrac{100(20^3)}{12}$
$I_1 = \frac{200\,000}{3} \, \text{mm}^4$

For the whole beam

$I = 6I_1 = 6(200\,000/3)$

$I = 400\,000 \, \text{mm}^4$

$\dfrac{M}{EI} = \dfrac{M_1}{E_1 I_1}$ where E_{1} = E

$\dfrac{M}{E(400\,000)} = \dfrac{M_1}{E(\frac{200\,000}{3})}$

$\dfrac{M_1}{M} = \dfrac{1}{6}$ *answer*

Part (b)

For 1 plank

$I_1 = \dfrac{100(10^3)}{12}$
$I_1 = \frac{25\,000}{3} \, \text{mm}^4$

For the whole beam

$I = 12I_1 = 12(25\,000/3)$

$I = 100\,000 \, \text{mm}^4$

$\dfrac{M}{EI} = \dfrac{M_1}{E_1 I_1}$ where E_{1} = E

$\dfrac{M}{E(100\,000)} = \dfrac{M_1}{E(\frac{200\,000}{3})}$

$\dfrac{M_1}{M} = \dfrac{1}{12}$ ) *answer*