What challenges made the use of iron slower to develop than copper?

Ineed to find the answer of this question because i dont understand it

He table below gives the atomic mass and relative abundance values for the three isotopes of element m. relative abundance (%) atomic mass (amu) 78.99 23.9850 10.00 24.9858 11.01 25.9826 what is the average atomic mass (in amu) of element m? 2.86 5.36 24.30 24.98

How many moles of oxygen gas are produced from the decomposition of six moles of potassium