blue balls=12
yellow balls=y
total=12+y
If P(blue ball)<2/5, then P(yellow ball) > 3/5

the least number of yellow balls could be > 3(12+y)/5 (note the sign greater, this will be used later in solution). Let's check each answer choice - apply POE (process of elimination)
a) y=17, 3(12+17)/5 isn't integer
b) y=18, 3(12+18)/5=18 fits but we need the value greater, which could be 19
c) y=19 correct
d) y=20, 3(12+20)/5 isn't integer
e) y=21, 3(12+21)/5 isn't integer

c

y/(12+y)>3/5

5y>36+3y
y>18

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12/(12+y)<2/5
Y>18