What are the most difficult concepts for students to understand?

Hemoglobin is the compound in red blood cells that carries oxygen to the body. the distribution of hemoglobin in women in g/dl of blood is approximately normally distributed with mean 14 and standard deviation 1. too little hemoglobin, and you're anemic. 2. too much, and (unless you live at high altitudes), you can have other problems.

A60-tooth gear is connected to a 72-tooth gear. if the smaller gear turns 12 times, how many turns does the larger gear make?

10111213141516lactic acid fermentation differs from ethyl alcohol fermentation in thato in ethyl alcohol fermentation co2 is also producedlactic acid fermentation can occur in all living thingsethyl alcohol fermentation can only occur in plantsonly lactic acid fermentation can produce more atp