You need to start calculating from the innermost bracket and if there are no signs to do any operation then you need to multiply
She has to buy both binders and notebooks. So, you have to take into account that she has to have both. The closest you can get to $20 while still getting notebooks, is to buy 4 binders. 4 times 4 equals 16. So, she can get 4 binders and 2 notebooks, because then, 2 times 2 equals 4 and 16 plus 4 equals 20.
((n-2)×180)/n = 156
(n-2)×180=156n
24n =360
n =15
360/n =36
n = 10
a)15 b)10