How many blue pens were there to start with?

180 blue   70 red

Step-by-step explanation:

b= blue      r = red

b+r = 250

r = (250-b)

If 40% is removed the rest is 60%

If the 70% is removed the rest is 30%

0.6b + 0.3(250-b)  = 129

0.6b + 75 - 0.3b = 129

  0.3b = 54

divide by 0.3

  b = 180

    250-180 =  70

   60% 0f 180 = 108

   30% of 70 = 21

   108 +21 = 129

51

step-by-step explanation:

73-22=51 the   mass number minus the amount of protons gives you the amount of neutrons