Not including tax, a total of 19 pens and markers cost $11.50. The pens cost $0.25 each, and the markers cost $0.75 each. Write the system of equations that could be used to solve for the number of pens (p) and the number of markers (m) bought.
p+m=_______
______p+ _______m=_______
p+m=19
0.25p + 0.75m = 11.50