Set the three ratio terms, say whether the ratio counts coins or rupees, and give the matching total. The walkthrough names the mismatch, converts the ratio with the magic numbers, finds the value of one part, and reports both the total coins and the total value with the cross-check.
coins 3 : 8 : 20, total value ₹372
12 value parts · x = 31 · 961 coins
Set the ratio to 2 : 3 : 4 by count with a total value of ₹372 to see the guard fire — that combination needs 165.33 rupee coins, so it is not a bag that can exist.