Susan and her sister Katie are making baby blankets to sell at a boutique. Susan has already completed 3 blankets and can finish 5 more blankets per day. Katie has already completed 5 blankets and can finish 4 more blankets per day. At some point, they will have completed the same number of blankets. Write a system of equations that would represent this problem if x represents the number of days and y represents the number of blankets.