唔識....! !!thx~.

Let starting salary = P
monthly increment = r
For A,
1st year income = 12P (P = 20000)
2nd year income = 12(P + r) (r = 2000)
3rd year income = 12(P + 2r)
4th year income = 12(P + 3r)
nth year income = 12[P + (n- 1)r]
so total income after n years = sum of each year = 12nP + 12r[1 + 2 + 3 + 4 +........+ (n -1)]
= 12nP + 12rn(n -1)/2
= 12nP + 6rn(n - 1)...............(1)
For B,
1st half year income = 6(P/2) = 3P
2nd half year income = 6[P/2 + r] = 3P + 6r
3rd half year income = 6[P/2 + 2r] = 3P + 12r
4th half year income = 6[P/2 + 3r] = 3P + 18r
2nth half year income = 6[P/2 + (2n - 1)r] = 3P + 6r(2n - 1)
so total income after n years = sum of each year
= 6nP + 6r[1 + 2 + 3 + ...... + (2n - 1)]
= 6nP + 6rn(2n -1)...... (2)
(1) = (2)
12nP + 6rn(n - 1) = 6nP + 6rn(2n - 1)
6nP = 6rn [ (2n -1) - ( n - 1)]
P = rn
so n = P/r = 20000/2000 = 10 years.



2009-10-08 07:52:48 補充：
Correction : For B, total income after n years = sum of each HALF years ( total 2n half years)