In how many different orders can you line up 8 cards on a shelf

Aquantity with an initial value of 3900 decays exponentially at a rate of 35% every hour. what is the value of the quantity after 0.5 days, to the nearest hundredth?

Hiroaki wants to find 3 3/4 - 7/8 explain why he must rename 3 3/4 to do the subtraction.

How do i simply this expression (quadratic formula basis) on a ti-84 or normal calculator?