Oral maintenance dose of Cefalexin?

t½ = 1 hour;   Clearance = 4.3 mL/min/kg;   F = 0.9

what is the ORAL maintenance dose (in mg) of cefalexin to achieve a steady-state plasma concentration (Css,av) of 5 mg/L for a 17 kg child.  The dosage interval should be 6  hours  (four  times  a  day).    Cefalexin  is  available  as  a  25  mg/mL  paediatric  mixture. The recommended dose must be in mg and be practical (Hint: to the nearest 25 mg).

Answer:   Dose = 150 mg

formulas that i used but can't arrive at the exact answer:

c1=c2e^-kt2-t1

t1/2=0.693/k

MD=Css,av x CL x t / F

Css,av=F x DM/CL x t

THANK YOU! :)

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