Out of the first 100 balls, Ralph was able to hit 2/5 of them and not able to hit 3/5 of them, 3/5 x 100 = 60 tennis balls Ralph didn't hit.
Out of the next 75 balls, Ralph was able to hit 1/3 of them and not able to hit 2/3 of them, 2/3 x 75 = 50 tennis balls that Ralph didn't hit.
Combined, Ralph was not able to hit 60 + 50 = <<60+50=110>>110 tennis balls Ralph didn't hit.
#### 110