jlgdr
A machinist's salary at a factory increases by $2,000 at the end of each full year the machinist works. If the machinist's salary for the fifth year is $39,000, what is the machinist's average annual salary for his first 21 years at the factory?
Solution: We are given that machinist's salary for the fifth year is \($39,000\). And the salary at a factory increases by \($2,000\) at the end of each year.
So, we know \(S_5=$39000\) and consequently:
\(S_4=$39000-2000=$37000\),
\(S_3=$37000-2000=$35000\),
\(S_2=$35000-2000=$33000\) and
\(S_1=$33000-2000=$31000\)
We can draw an inference that
we are dealing with arithmetic progression (AP) with first term \(a=31000\) and common difference \(d=2000\)We are asked the average of annual salary for his first 21 years. This means we are asked the \(\frac{sum of first 21 terms}{21}\)
We can use the formula of sum of AP to get sum of 21 terms. Sum of 21 terms \(= \frac{n}{2}[2a+(n-1)d]=\frac{21}{2}[2\times 31000+20\times 2000]= \frac{21}{2}[102000]=21\times 51000\)
Thus average \(= \frac{21\times 51000}{21}=$51000\)
Hence the right answer is \($51,000\)