A mosquito sucks 20 drops of blood every time it feeds on someone. If there are 5000 drops per liter and you have to lose 3 liters of blood to die, how many mosquitoes would have to feed on you to kill you?
First find the total number of liters a mosquito sucks in one feeding: 20 drops / 5000 drops/liter = 
<gadget id="calculator">20/5000</gadget>
<output>1/250 = around 0.004</output>
.004 L/feeding.
Then divide the lethal amount of blood by the amount of blood one mosquito can drain: 3 L / .004 L/feeding = 
<gadget id="calculator">3/.004</gadget>
<output>750</output>
750 feedings.

<result>750</result>