$1)$ A merchant could sell one model of digital cameras at list price for $$130.$ If he had five more cameras, he could sell each one for $$13$ less and still receive $$130.$ Find the list price $($in dollars$)$ of each camera. $2)$ Working together, Sarah and Heidi can clean the garage in $6$ hours. If they work alone, it takes Heidi $9$ hours longer than it takes Sarah. How long in hours would it take Heidi to clean the garage alone? $3)$A merchant could sell one model of digital cameras at list price for $$130.$ If he had five more cameras, he could sell each one for $$13$ less and still receive $$130.$ Find the list price $($in dollars$)$ of each camera. $4)$A county newspaper earns a profit of $$20$ per year for each of its $4,000$ subscribers. Management projects that the profit per subscriber would increase by $1$ for each additional subscriber over the current $4,000.$ How many subscribers are needed to bring a total profit of $$210,900?$