Most reasonably sized commercial aircraft rely on jet engines for propulsion. Upon landing these aircraft use a number of means to come to a stop with the two most important being (I believe) mechanical brakes and jet engine thrust reversal. This reverse thrust is generated by opening some small doors on the outside of the engine (shown below). How efficient is the thrust reversal process? For specificity, let's define the efficiency to be reverse thrust divided by forward thrust for a given engine speed.
The answer depends on factors like the bypass ratio of the engine and the design of the thrust reverser, e.g. bucket doors at the back of the engine or vanes to deflect the bypass airflow from the fan.
By the OP's proposed measurement (reverse thrust / forward thrust), the efficiency is also strongly dependent on the engine speed. At maximum thrust the reverser may be 50% or 60% efficient, but at lower power levels it may be less than 10%.
I agree that reverse thrust is not nearly as effective ("efficient" might not be the correct term) as forward thrust, and I'm surprised that it could be as high as 50%. The previous responder (alephzero) did say that it is probably only that efficient at full power, and in my experience full reverse thrust is rarely used - very noisy. However, on big advantage of using reverse thrust is that it eliminates the residual forward thrust that is otherwise present. Just that can make a big difference on a slippery runway.