posted by Kumar .
The atomic mass of Eu is 151.96. There are only two naturally occurring isotopes of europium, 151Eu with a mass of 151.0 and 153Eu with a mass of 153.0. The natural abundance of the 131Eu isotope must be approximately
Surely you made a typo in the last sentence and meant 151Eu and not 131. I am assuming you meant 151.
Let X = fraction of 153, then
1-X = fraction of 151.
151.0(1-X) + 153.0(X) = 151.96
Solve for X and 1-X which gives the fraction and that times 100 gives percent. .