Step 1: Fix the quantities of Q, R, S, T using one unknown.
Take T's quantity as $n$. Then S $= 2n$, R $= 3n$, Q $= 4n$, so these four items total $n + 2n + 3n + 4n = 10n$.
The remaining items are P, so P's quantity is $250 - 10n$.
Step 2: Check revenue against the known average.
The 250 items sold for an average of Rs. 60 each, so the total money collected is $250 \times 60 = 15000$ rupees.
Writing each product's contribution: P gives $100(250 - 10n)$, Q gives $50 \times 4n = 200n$, R gives $40 \times 3n = 120n$, S gives $60 \times 2n = 120n$, T gives $60n$.
Step 3: Add everything and solve.
Adding all contributions: $25000 - 1000n + 200n + 120n + 120n + 60n = 15000$, so $25000 - 500n = 15000$, giving $n = 20$.
So P's quantity is $250 - 200 = 50$, and a quick check confirms the revenue: $50(100)+80(50)+60(40)+40(60)+20(60) = 15000$.
Final Answer:
The quantity of P sold works out to 50 units.\[ \boxed{P = 50} \]