Multiple choice general knowledge math & puzzles

Gold is 19 times as heavy as water and copper is 9 times as heavy as water. In what ratio should these be mixed to get an alloy 15 times as heavy as water?

  1. 1:1

  2. 2:3

  3. 1:2

  4. 3:2

Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Using allegation method: Gold is 19 parts, copper is 9 parts, alloy is 15 parts. Gold contributes 4 parts above 15 (19-15), copper contributes 6 parts below 15 (15-9). The ratio is the inverse: gold:copper = 6:4 = 3:2. Alternatively: 19g + 9c = 15(g+c), so 19g + 9c = 15g + 15c, giving 4g = 6c, so g:c = 6:4 = 3:2.