Answer: 9 strikeouts
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Explanation
Lets say he had 3 strike outs and 4 other at bats. We get the ratio 3:4 describing the connection between strikeouts and other at bats. When I say "other at bats", I mean "anything that is not a strikeout". So this includes hitting the ball and getting a walk.
We can see that he has 3+4 = 7 at bats total (ie 7 attempts total). This is not 21, but it's a factor of 21
We can multiply that 7 by 3 to get 7*3 = 21
Multiply the other parts of the ratio 3:4 by 3 like so
The ratio 3:4 is equivalent to 9:12 after tripling each part.
He has 9 strikeouts and 12 other at bats, giving 9+12 = 21 attempts total. This helps confirm we have the right ratio.
