$F_1 = p_{cg1} A_1$

$F_1 = [ \, 9.81(0.8)(\frac{1}{2}a) \, ](ab)$

$F_1 = 3.924a^2b$

$F_2 = p_{cg2} A_2$

$F_2 = \left[ \, 9.81(0.8)(a) + 9.81 \left( \dfrac{a}{2} \right) \, \right] (ab)$

$F_2 = 12.753a^2b$

Required ratio

$\dfrac{F_2}{F_1} = \dfrac{12.753a^2b}{3.924a^2b}$

$\dfrac{F_2}{F_1} = 3.25$ ← *answer*