calculate the atomic weight: 75% have a mass of 5g 25% have a mass of 15g

Explanation:

Step1: Convert percentages to decimals

$75% = 0.75$, $25%=0.25$

Step2: Use the weighted - average formula

Atomic weight $=0.75\times5 + 0.25\times15$ $=3.75+3.75$ $ = 7.5$ g

Answer:

$7.5$ g