If 75.0% of the isotopes of an element have a mass of 35.0 amu and 25.0% of the isotopes have a mass of 37.0amu what is the atomic mass of the element
a) 35.0amu
b)35.5amu
c)36.0amu
d)37.0amu

 Calculating average atomic mass is exactly like calculating a weighted average. Perform the following calculation: 

(mass1)(percentage1) + (mass2)(percentage2) = average atomic mass 

(35.0)(0.75) + (37.0)(0.25) = average atomic mass 

Make sure your percentages are in decimal form for this calculation. 

One of the other answers given is correct, though the explanation is lacking a bit. Two of the answers can be eliminated immediately: 35.0 amu and 37.0 amu cannot be the average. If the mixture of isotopes was 50% and 50%, then 36.0 amu would be correct; however, the mixture is 75/25. This leaves only one possible answer choice.

D)can be increased in a closed system