Please help me i am confused

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?

Use the method of cylindrical shells to find the volume v generated by rotating the region bounded by the curves about the given axis. y = 7ex, y = 7e^-x, x = 1; about the y-axis

According to the general equation for conditional probability if p(anb)=3/7 and p(b)=7/8 what is p(a i b)