Significant figures in a calculation of atomic mass from isotopic abundance
First, for reasons I'll point out later this is a poorly made-up problem which was only designed to test significant figures, not to teach anything real about chemistry.
Second there are no special rules for &quot;word problems&quot; using significant figures.
The gist is that significant figures are a simple method to do error propagation in calculations. It is essentially based on the notion that the error is +/- 1/2 of the last significant digit, as if the value had been rounded to that digit. So 151 means that the value should be between 150.5 and 151.5.
The whole point of significant figures is to prevent you from dividing 151 by 3.00 and getting some ridiculous precision like 50.333333333333333333333333333333.
Now on to the problem. 151 and 153 are mass numbers for the europium  isotopes. Mass numbers are poor values to use for the isotope  masses. If fact the known masses are 150.9198502(26)  and 152.9212303(26). The numbers in parenthesis give the standard deviation of the measurement. So for 150.9198502 the standard deviation is 0.0000026.
Likewise according to the Wikipedia page, $\ce{^{151}Eu}$ has an abundance of 0.4781(6) and $\ce{^{153}Eu}$ has an abundance of 0.5219(6). The point is that the isotopic variation in samples taken from all over the earth limits the precision of the atomic mass for any element, not the mass of the isotopes which are know with much greater precision.
The same Wikipedia page linked earlier gives the atomic mass of europium as 151.964(1) which is inconsistent with the given percentages. I do not know the how/why there are so many more significant figures in the atomic mass (and I am too lazy to turn this answer into a research project to figure that part out...).