What is the LCD of 5/6 and 7 /8

Ineed to learn how to solve this problem

17 in long 1.3 ft wide and 8in high what is the volume

Afarmer has a large field that is x feet in length. he wants to fence in a rectangular section in the middle of the field , leaving a length of 100 feet of open field behind each end of the fenced rectangle.he also wants the width of the fenced-in space to be 100 feet less than its length. find the expressions to represent the length and width of the fenced section of the field