how do you compute this using clark's rule for pediatric dosing?

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Clark's Rule for Dosing in Pediatrics states the following:

The procedure or calculation is to take the child's weight in pounds, divide by 150 lb, and multiply the fractional result by the adult dose to find the equivalent child dosage.

Adult dose in this case = 500mg

child weight = 40 lbs

Divide the weight by 150

--> 40/150 = 0.267

--> now multiply the fractional answer you just calculated by the initial adult dose to get the equivalent child dose

--> 0.267 x 500 = 133.33mg should be the approximate equivalent child dose to a 500mg adult dose

hope this helps !