Note:
(So the question implies that we must assume that there are no transmission errors)
t_{ACK} ------ << 1 t_{data}
t_{ACK} ------ = 0 t_{data}
t_{data} + t_{prop} + t_{ACK} + t_{prop} = t_{data} + t_{ACK} + 2×t_{prop}
t_{data}
t_{data} Channel efficiency = --------------------- t_{data} + t_{ACK} + 2 t_{prop} t_{data} ~= --------------- t_{data} + 2 t_{prop}
(t_{ACK} is very small and is usually discarded in the approximation)