Never use the annuities formula in Further. Use the TVM on your calculator instead. It is easier and less susceptible to errors in typing and calculation.
TVM can do CI formula questions and more. Use the CI formula if there are no repayments made during a loan or for reducing balance depreiation. Use the TVM for any question where there are regular paments being made.
To decide to use either of these, look for these words or phrases in the question. Exponential growth, compounding, reducing balance, increasing balance, compounding or an increase or decrease by a percentage at each calculation of interest. If interest at each calculation is greater or smaller than the interest in the previous calculation, this is also an indicator that CI is involved. Also if a graph of the balance of an investment shows exponential growth or, the balance of a loan or depreciation shows exponential decay, these also apply to the CI formula or TVM.
See 2013 paper 2 questions 2a, 2b, 3d and 4. Also 2011 paper 2 questions 2 and 3b and 3c
Sometimes you will have to use TVM twice in a question. Look for any change to the interest rate or to the payments during a loan or investment in the question. See 2011 paper 2 question 4
The SI formula is used if you see the words simple interest or flat rate or if the same amount of interest is added every time it is calculated. If there is a grapgh, look for a linear pattern. See 2013 paper 2 questions 1a and 1b
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