Question: create math page exercises to print for my daughter

I'm creating a table of random math problems for my daughter to print out.  How do I get more columns?

a := rand(1 .. 15):
b := ["+", "-", "x", "/"]:

for i to 15 do
  print(cat(a(),b[rand(1..nops(b))()],a(),"_____")):
  print(``):
end do;

                                  13x4_____

                                  4-14_____

                                  7+15_____

                                  2-5_____

                                 13-10_____

                                  3+6_____

                                  5+13_____

                                  14+4_____

                                 15x10_____

                                  14x4_____

                                  7x1_____

                                  7+2_____

                                  1/11_____

                                  7+11_____

                                  8+14_____