Let the cost price of the whole seller be x.

The whole seller got a profit of 4%, so the selling price for the whole seller would be:

x + 0.04x = 1.04x

Sara bought 5kg potatoes from the whole seller, so the cost price for Sara would be:

5x

The shopkeeper got a profit of 4%, so the selling price for the shopkeeper would be:

1.04(5x) = 5.2x

Sara bought the potatoes from the shopkeeper for RS. 390, so we can set up the equation:

5.2x = 390

Solving for x, we get:

x = 75

Therefore, the cost price of the whole seller was RS. 75.