I realize that I am a little late on this one, but I did want to make a point. Mathyperson is absolutely correct in his approach, however the nature of this question suggests that the asker has limited understanding of fractions at this point. I would offer the additional explanation of:

"1 and 1/3" is a mixed fraction meaning that it is a composition of a whole number and a fraction.

To add or subtract with fractions all terms must be in fractional form and have identical denominators. The top number of the fraction is called the numerator and the bottom term is called the denominator. 1 1/3 is pronounced "one and one-third". To convert this mixed fraction into a regular fraction you need to multiply the whole number times the denominator then add the numerator and use this answer as the new numerator atop the previous denominator. Easier done than said.

1 1/3 = ( 1 X 3 + 1 ) / 3 = 4/3 or four-thirds

To subtract 7/8 from 4/3 you must make both denominators the same. As Mathsyperson stated the lowest common denominator is 24. There are various ways in which to determine this, but the simplest is simply to ask first if the smaller denominator goes into the larger one evenly. If it does, then you only have to alter one fraction by multiplying both the numerator and denominator of the smaller one by that amount. If it does not, then you will have to determine what the smallest number that both denominators go into evenly and then multiply each fraction appropriately.

The two denominators are 3 and 8. Since three does not go into eight evenly you have to try multiples of 8. 3 does not go into 16 either, but it does go into 24. So both denominators must be converted to 24 and whatever you do to the denominator you must also do to the numerator or you will change the ratio of what the fraction represents. Again, this is all much easier to do than to explain.

4/3 - 7/8 = 32/24 - 21/24 = 11/24

Sorry if this was a little long, but I just wanted to be clear in case you were unsure about other aspects of this problem.