Write a system of equations to describe the situation below, solve using elimination, and fill in the blanks.

Two coworkers picked up some writing instruments at the office supply store. Sarah selected 9 boxes of pencils and 4 boxes of ballpoint pens, paying $25. Next, Heather spent $40 on 4 boxes of pencils and 9 boxes of ballpoint pens. How much does a box of each cost?
A box of pencils costs $, and a box of pens costs $.

Anew test for ra is being developed. the test is administered to 750 patients with ra and 750 without ra. for the sake of the question, assume the prevalence of ra in this population is 50%. of the 750 patients with ra, 575 have a positive result. of the 750 patients without ra, 150 have a positive result. what is the positive predictive value of the new test? a575/(575+150) b. 600/(600+ 150) c. 575/(575+125) d. 600/(600+ 125)