The human body is 65% oxygen, 18% carbon! and 10% hydrogen. the rest is other elements. what percent is composed of other elements?

Well, simply young soul, you have to add up everything you have so far and subtract that by 100% because we know 100%= 100/100 just like 65%= 65/100, 18%= 18/100 and 10%= 10/100.
    65
    18               If all that added together is 93 we can put this over 100 just like
  +10               all the other ones.  93/100= 93%
            Now take the whole number (100) and subtract
   93               100-93= 7 which is 7%      
                      7% of the human body is made up of other elements. Hopes this 
                      helps!! :D