Q.A trader owes three bills — ₹1,000 due 10 days after a chosen base date, ₹1,500 due 25 days after the base date, and ₹2,500 due 55 days after the base date. Find the average due date, measured in days after the base date.

Compute the amount-weighted average of the day-counts, exactly as in the earlier worked example.

$\sum a_it_i=1{,}000(10)+1{,}500(25)+2{,}500(55)=10{,}000+37{,}500+1{,}37{,}500=1{,}85{,}000$. …

Here the three bills are $a_1=₹1{,}000$ at $t_1=10$ days, $a_2=₹1{,}500$ at $t_2=25$ days, and $a_3=₹2{,}500$ at $t_3=55$ days from the chosen base date.

Step 1 — Compute each product $a_it_i$.

Step 2 — Sum the products and the amounts. …

Forgetting to include a bill's own day-count in BOTH the numerator (as $a_it_i$) and the denominator (as part of $\sum a_i$) — every bill contributes its amount to the …