Respuesta :
Answer:
B= black pens
X= blue pens
R= red pens
T=total
B+x+r= t
170+120+r=350
Not sure this is correct but you can solve it like this
Total Number of Pens - (Black Pens + Blue Pens)
350 - (170 + 120)
350 - 290 = 60 pens
350 - (170 + 120)
350 - 290 = 60 pens