Calculate the atomic mass of chromium it’s composition is 83.79% with a mass of 51.94 amu; 9.50% with a mass of 52.94 amu; 4.35% with a mass of 49.95 amu; 2.36% with a mass of 53.94 amu

Convert the percentages into decimals (you can do that by dividing the percent by 100), then multiply that by its corresponding mass to find its relative amount/ contribution to the atomic mass of chromium. After doing so, add all of the obtained values together to get the average mass.

83.79% = .08379
9.50% = .095
4.35% = .0435
2.36% = .0236

Average mass of chromium = 0.8379(51.94) + 0.095( 52.94) + 0.0435(49.95) + 0.0236(53.94)

52amu

P.S. never forget units

The mass has escaped into a gas which you can not see so the law stays true.