X/20 = 18/10 Multiply both sides by 20 to isolate X and then do division. X is the total weight.
For all practical purposes, pure gold is 24k. 18k is 18/24 gold and 6/24 another metal. Multiply 20 by 18/24 to find amount of gold you have. Now subtract that from the total weight. The rest is another metal. Now multiply 20 by 6/24 to find aout how much other metal you had to start with. Subtract that from your last answer and that will be the amount of another metal you must add to your 20 grams of 18k gold to bring it down to 10k gold (10/24)
For all practical purposes, pure gold is 24k. 18k is 18/24 gold and 6/24 another metal. Multiply 20 by 18/24 to find amount of gold you have. Now subtract that from the total weight. The rest is another metal. Now multiply 20 by 6/24 to find aout how much other metal you had to start with. Subtract that from your last answer and that will be the amount of another metal you must add to your 20 grams of 18k gold to bring it down to 10k gold (10/24)