The ratio is the best way to do these problems. Also, work lots of them.
2.2lbs = 1.0kg so 78/2.2 = 35.5kg
750mg/5cc = 150 mg/1cc
225 * 35.5kg = 7987.5mg
------ = -----
150x = 7987.5
x = 7987.5/150
x = 53.25cc
I put in all the steps. I also took the liberty to change mcg to mg, in case of a typo. If you need to change mg to mcg multiply mg * 1000.
Good Luck. !!
Remember we've all been there and some of us cant do math. And it may just take some extra practice. With your grades, I's sure you know how to study. Also, see if your instructor will cut you a break and let you know what are the most important conversions to know. If there is only 5 problems, they probably wont put drams or grains on it.