let's bring both fractions to the same form ok?
\(\frac{1}{6} = \frac{1*2}{6*2} = \frac{2}{12}\)
\(\frac{3}{4} = \frac{3*3}{4*3} = \frac{9}{12}\)
so, if every day he paints
\(\frac{2}{12}\)
then after the second day he will have
\(\frac{4}{12}\)
after the third
\(\frac{6}{12}\)
after the forth
\(\frac{8}{12}\)
and in the middle of the firth day he will have
\(\frac{9}{12}\), or 3/4!
(alternatively 9/2=4.5, so he needs 4 and a half days)