Four milliliters of ink a is added to 10 milliliters of ink b, forming a mixture of inks. ink a contains 10% blue pigment. ink b contains an unknown percentage of blue pigment. the mixture of the two inks contains 35% blue pigment.

We will assume that B contains higher percentage of blue pigment than the mixture ( because A contains lower). First we will subtract:
35-10=15 and then: x-35
So we will make the proportion:
4 : 10 = (x-35) : 15
4 * 15 = 10(x-35)
60=10 x- 350
10 x =410
x = 41%
Ink B contains 41 % of blue pigment.