A medium sized food retailer currently receives $800$ trays of inventory each day on its morning delivery. It employs five members of staff to replenish the shelves with this stock in a two$-$hour time slot, prior to the store opening.

Towards the Christmas peak trading period, the number of trays increases by$\mathrm{ 5}0\%$ and the staff levels increase by $20\%.$ 

a$.$ Calculate the change in labour productivity per hour, between the pre Christmas and Christmas peak trading periods, during the morning replenishment of stock. Show your workings.