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If you want to do this exactly, you will need to know the masses of the isotopes; i.e., they are not exactly 25, 26, and 24. The problem is done this way.
Let x = abundance 25
Then 1.00-0.79-x =0. 21-x = abundance 26.
(0.79*24) + (x*25) + (0.21-x)*26 = 24.3
You will need to substitute the actual masses for 24, 25, & 26 in the above as well as the actual mass of the 24.3 total. Post your work if you get stuck.