Answer
The time he took to assemble the fifth computer is 20 minutes (round to the nearest minute).
The total time he took to assemble the first five computers is 123 minutes (round to the nearest minute).
Work Step by Step
The situation can be represented by a geometric sequence with $a_{1} = 30$ and $r = 0.9$, which the general term $a_{n} = 30(0.9)^{n-1}$ where n is the number of computer to assemble
The time he took to assemble the fifth computer (n = 5) is
$a_{5} = 30(0.9)^{5-1}$ = 20 minutes (round to the nearest minute)
And, the total time he took to assemble the first five computers is
$\sum\limits_{n=1}^5 a_{n} = \sum\limits_{n=1}^5 30(0.9)^{n-1}$
= $\frac{30(1 - 0.9^5)}{(1 - 0.9)}$
= $123$ minutes (round to the nearest minute)
