To find the primary current of the step-down transformer, we follow these steps:

1. Calculate the Power Input:
 The efficiency (\( \eta \)) of the transformer is given by:
 \(\eta = \frac{\text{Power Output}}{\text{Power Input}} \times 100\)
 Rearranging gives:
 \(\text{Power Input} = \frac{\text{Power Output}}{\eta}\)

 Given:
 - Power Output = 50 W
 - Efficiency = 80\% = 0.80

 Thus,
 \(\text{Power Input} = \frac{50 \, \text{W}}{0.80} = 62.5 \, \text{W}\)

2. Calculate the Primary Current:
 The power input can also be expressed in terms of voltage and current:
 \(\text{Power Input} = V_p \times I_p\)
 Where:
 - \( V_p \) = primary voltage (given as 200 V)
 - \( I_p \) = primary current (what we want to find)

 Rearranging gives:
 \( I_p = \frac{\text{Power Input}}{V_p}\)

 Now substituting the values:
 \(I_p = \frac{62.5 \, \text{W}}{200 \, \text{V}} = 0.3125 \, \text{A}\)

Thus, the primary current of the transformer is: 0.3125A