How to solve this?

Question

On his first morning in a television assembly factory, a trainee
takes 0.8 times as long to assemble each television as he
took to assemble the one before. If it took him 40 minutes
to assemble the first television, find how long it takes him to
assemble the fourth television. Find the total time he takes
to assemble the first four televisions (round to the nearest
minute).

Answer 1

a_n = a₁r^n-1

a₄ = 40(0.8)³

a₄ = 40(0.512)

a₄ = 20.48

 

Sn = a₁[(1 - rⁿ) / (1 - r)]

S₄ = 40[(1 - 0.8⁴) / (1 - 0.8)]

S₄ = 40[(1 - 0.4096) / 0.2]

S₄ = 40[0.5904) / 0.2]

S₄ = 40(2.952)

S₄ = 118.08