750 gold as white gold content 75% of pure gold.

15:2:3 —> total 20u
Gold percentage = 15/20 x 100% = 75%.

Hope my explanation is correct ;)

Irah, the method which the tutor has used is what is called as a “weighted average”, in the sense that different percentages of different materials are used (and so you cannot just take the average directly).

Say for example, out of 40 students, 1 student gets 100 marks and the rest gets 0 marks. The average mark in the class cannot be 50 even though 50 is the average of 0 and 100, since there are so many more students scoring 0 than 100.

The method in which you use to calculate the average for this case is called the weighted average.

For your information, the relative molecular mass of an element is calculated in the same manner too. Chlorine, with an Mr of 35.5, gets its value from an almost 25% abundance of 37-Cl and an almost 75% abundance of 35-Cl (and an insignificant abundance of other Cl).

750 probably gets its number from 75 percent gold, but I wonder why the naming is then 750 and not 75.

Maybe 750 represents “75.0 percent gold”.

Yes, it is called 750 gold because it contains 75.0% gold. In real life, you will see that some gold shops in Singapore advertise 999, 925 and 916 gold.

They represent gold of 99.9% , 92.5% and 91.6% purity respectively.


Additional info : (if you're interested)

'Pure gold is defined as 24 karats or having a fineness of 1000: one karat equal to 41.66 fineness. A 22 karat gold-alloy, for example, has 22 parts gold and 2 parts other metal (usually copper), and has fineness of 916.66 (is 91.666 percent pure gold). Similarly, an 18 karat gold-alloy is 75 percent pure gold'

For 22 karats, 22/24 x 100%
≈ 91.666 %

For 18 karats, 18/24 x 100% = 75.0%

You will often see gold advertised as 24K, 22K and 18K as well.

Thank you so much, everyone for making me understand the reasons behind the statement and allowing me to learn something from real-life scenarios! :)