Monthly installments/payments, P = C*D
C = cost = $2,469
D = (i/12)/[1-(1+i/12)^-48] = (0.021/21)/[1-(1+0.021/12)^-48] = 0.0217
Therefore;
P = 2469*0.0217 = $53.67
Total interest paid, I = Total amount paid - Cost
Total amount paid = P*48 = 53.67*48 = $2,576.30
Then,
I = 2576.30-2469 = $107.30