Ok, so 1 pound is equal to 0.453 kg. For a child weight of 42.7 then that makes 42.7 times 0.453, giving you 19.3431 kg. the density of the suspension is equal to 60 mg/mL.
Remember Volume=Mass/Density and the mass is 19.3431 kg which is equal to 19343.1 mg.
(19343.1mg)/(60mg/mL) equals 322.385 mL
I hoped this helped.