$($b$)$ A souvenir store can obtain a particular souvenir water bottle from a custom merchandise company at a unit cost of RM $5.$ Currently, the store sells the water bottles at a price of RM $35$ per unit and has been selling $210$ units a month at this price. The store plans to lower its price to boost sales and estimates that for each RM $1$ reduction in the price, an additional $15$ water bottles will be sold each month. At what price should the souvenir store sell the water bottles to generate the greatest possible profit?