= Solution
The target collision <energy> requires $175$ GeV per lepton beam. Its initial <energy> after the half-linac is $62.5$ GeV, so each lepton must gain $112.5$ GeV. Define the <transformer ratio> as accelerating field experienced by the witness divided by decelerating field experienced by the driver. If the useful stage <length> exhausts the driver's $62.5$ GeV while delivering the required gain, then
$$
\boxed{k_T=\frac{112.5}{62.5}=1.8.}
$$
This matches depletion to <acceleration> and avoids leaving unused driver <energy> at the witness target. It is an ideal energy-transfer estimate, assuming suitable loading and longitudinal field profiles; a field ratio alone does not specify a practical wake or its efficiency.
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