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As described in the problem, for each extra room rented there will be $3 off the base price of 70$
(a) Now, when a family rents four rooms. That means they have rented 3 extra rooms.
$3 off for each extra room so discount for 3 extra rooms = 3 * 3$ = 9$
So price per each room is base price - discount i.e. 70$ - 9$ = 61$
Price for four rooms = 4 * 61$ = 244$
(b) Formula for computing rate you charge for one room when n rooms are rented is:
Rate per room = 70$ - ( (n-1) * 3$)
Above formula can be solved to get 73$ - 3n$
Above formula is derived using below logic
Rate per room = Base Price - (Number of extra rooms * 3$)
Note: In case you are asked to find out total price for n rooms, then you need to multiply price per room with n.
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