Well since you didn't provide the choices, I'll have to make some assumptions... we'll call the red pens R, the black pens B and the Blue pens x. If total cost = T then We can say...
T=R(2.50)+B(2.50)+x(2.50)
It could also be shown as
T = R + B + x(2.50)
Another way that we could have approached it since there is no difference in price between colors is just say each pack of pens (no matter the color) = P... so...
T = P (2.50)