Algebra

i think its very simple, but i can't seem to get it right..

Find the last digit when 13 is raised to 655.

A pile of creosoted poles contains 1275 poles in layers so that the top layer contains one pole and each lower layer contains one more pole than the layer above. How many layers are in the pile? ( SHOW YOUR SOLUTION PLEASE)

Factored form of X³ + 3x² – 4x - 12

sqrt(x-sqrt(1-x)) = 1-sqrt(x)

solve for x

the question in the book is write the first four terms of the g.s. , if the sum of the first and fourth terms is 195 and the sum of the second and third terms is 60.
asub1+asub1r^3=195 (1)
asub1r+asub1r^2=60 (2)
Dividing (1) by (2)
r=1/2; r=4
Substitute in (2)
asubn=192; asub1=3
Therefore, the four terms are 3, 12, 48, 192.
My question is how did they get r=1/2 and r=4?

If a skydiver with a surface area (into the wind) of 2m(squared) ... falls at a steady speed of 55m/s with a force of 900N what is the pressure on her?