Lisa, an experienced shipping clerk, can fill a certain order in 10hrs. Tom, a new clerk , needs 21 hrs to do the same job. Let x represent the number of hrs. it takes Lisa and Tom to fill the order working together. Determine an expression in terms of x that represents how much of the order will be completed in x hours. An expression in terms of x that represents how much of the order will be completed in x hours is
(simplify your answer. Use integers or fractions for any numbers in the expression.)