Alf would prefer to borrow from his parents because their rates are more favourable.
The future value of the loan if he borrows from his parents is given by this formula: A(1 + r)^n
Where:
A = amount
R = interest rate
N = number of years
$15,000(1 + 0.0355)^4 = $17,246.13
The future value of the loan if he borrows from the local bank is given by this formula: : A x e^r x N
Where:
$15,000 x 2.7182818^0.035 x 4 = $62,137.18
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