The earth's crust comprises a number of chemical elements. Here are a few along with the abundance as a percentage:
Iron 5.63%
Calcium 4.15%
Sodium 2.36%
Magnesium 2.33%
Potassium 2.09%
When asked which two elements make up seven percent of the earth's crust, we must find which two numbers (of the combinations listed) sum to 7. The closest sum is calcium and sodium, whose percentages sum to 6.51%.
