Tom and Jerry each start a new job. Tom is paid $30,000 in the first year, and in every subsequent year, his annual pay will increase by $1,500. Jerry is paid $25,000 in the first year, and in every subsequent year, his annual pay will increase by 5%.
If they both start working at the beginning of 2013,

  1. (i)

    what is the first year in which Jerry will be paid more than Tom?

  2. (ii)

    at the end of which year will Jerry's total income first exceed Tom's total income?

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Worked solution

4(i)

1. Express Tom's annual pay in the $n$th year

Let TnT_n be Tom's pay in the nnth year. Since his pay increases by a constant $1,500 each year, it forms an arithmetic progression with first term a=30000a = 30000 and common difference d=1500d = 1500.
Tn=30000+1500(n−1)T_n = 30000 + 1500(n-1)

2. Express Jerry's annual pay in the $n$th year

Let JnJ_n be Jerry's pay in the nnth year. Since his pay increases by 5% each year, it forms a geometric progression with first term a=25000a = 25000 and common ratio r=1.05r = 1.05.
Jn=25000(1.05)n−1J_n = 25000(1.05)^{n-1}

3. Formulate the inequality to find when Jerry's pay exceeds Tom's

We require Jn>TnJ_n > T_n:
25000(1.05)n−1>30000+1500(n−1)25000(1.05)^{n-1} > 30000 + 1500(n-1)

4. Solve the inequality using a Graphing Calculator (GC)

This mixed exponential-polynomial inequality cannot be solved algebraically. Using a graphing calculator to generate a table of values for TnT_n and JnJ_n:
When n=16n = 16, T16=52500T_{16} = 52500 and J16≈51973.11J_{16} \approx 51973.11, so J16<T16J_{16} < T_{16}.
When n=17n = 17, T17=54000T_{17} = 54000 and J17≈54571.76J_{17} \approx 54571.76, so J17>T17J_{17} > T_{17}.
The smallest integer nn is 1717.

5. Determine the actual calendar year

The 1st year (n=1n=1) is 2013.
The 17th year is 2013+17−1=20292013 + 17 - 1 = 2029.
Therefore, the first year Jerry will be paid more than Tom is 2029.

Answer

2029

4(ii)

1. Formulate an expression for Tom's total income after $n$ years.

Tom's annual pay forms an arithmetic progression with first term a=30000a = 30000 and common difference d=1500d = 1500.
Let STS_T be his total income after nn years.
ST=n2[2(30000)+(n−1)(1500)]S_T = \frac{n}{2}[2(30000) + (n-1)(1500)]
ST=750n2+29250nS_T = 750n^2 + 29250n

2. Formulate an expression for Jerry's total income after $n$ years.

Jerry's annual pay forms a geometric progression with first term a=25000a = 25000 and common ratio r=1.05r = 1.05.
Let SJS_J be his total income after nn years.
SJ=25000(1.05n−1)1.05−1S_J = \frac{25000(1.05^n - 1)}{1.05 - 1}
SJ=500000(1.05n−1)S_J = 500000(1.05^n - 1)

3. Set up the inequality and find the smallest integer $n$ using a graphing calculator.

We require Jerry's total income to exceed Tom's total income:
SJ>STS_J > S_T
500000(1.05n−1)>750n2+29250n500000(1.05^n - 1) > 750n^2 + 29250n
Using the table function on a graphing calculator:
When n=25n = 25, ST=1200000S_T = 1200000 and SJ≈1193177S_J \approx 1193177 (so SJ<STS_J < S_T)
When n=26n = 26, ST=1267500S_T = 1267500 and SJ≈1277836S_J \approx 1277836 (so SJ>STS_J > S_T)

The inequality is first satisfied when n=26n = 26.

4. Determine the corresponding calendar year.

The 1st year corresponds to 2013.
The 26th year corresponds to 2013+26−1=20382013 + 26 - 1 = 2038.

Jerry's total income will first exceed Tom's at the end of 2038.

Answer

2038

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