Let x be the number of races he have to win for his winning ratio to be 70% or 0.7
so,
(11+x)/(17+x)= 0.7
=> 11 +x = 0.7(17+x)
=> 11 +x = 11.9 +0.7x
=> x - 0.7x = 11.9 -11
=> 0.3x = 0.9
=> x = 0.9/0.3
=> x =3
So he have to win 3 more races. Or (11+3) = 14 races out of (17+3) = 20 races.