Use signed numbers to solve.

a helicopter descended 1200 feet, rose 800 feet, and then descended 450 feet.

what was the net gain or loss in altitude?

a.lost 2450 ft

b.lost 850 ft

c.gained 850 ft

d.gained 2450 ft

We are given the heights and the directions of the displacements of the helicopter. We assign going up or gaining of height as positive and going down or losing as negative. In this case, -1200 + 800 - 450 = - 850 feet. Answer is B

Let us first concentrate on the information's already provided. 
First case
The helicopter descended 1200 feet, so = - 1200
Second case
The helicopter rose 800 feet, so = 800
Then
The position at which the helicopter is now = -1200 + 800
                                                                     = - 450 feet
Third case
The helicopter again feel by 450 feet = - 450
Then
Position of helicopter = - 400 - 450
                                   = - 850
From the above deduction, we can say that the helicopter lost 850 feet. The correct option among all the options that are given in the question is option "B".

Since it is below, it should be negative, so -18