Question

White gold is an alloy that typically contains 45.0% by mass gold and the remainder is platinum. If 154 g of gold are available, how many grams of platinum are required to combine with the gold to form this alloy?

Answer 1

The percentage of gold in alloy by mass is 45%

That means that platinum makes up (100% - 45%) = 55% of the alloy by mass.

Therefore make your calculations as follows: 

If 45% translates to 154g (of gold)
What mass does 55% translate to?

45% = 154
55% = 154 × 55/45

          =  188.22 grams

Therefore you will need 188.22 grams of platinum to make this alloy that contains 154g.