Two angles have a sum of 90. the measures of the angles are in a ratio of 2: 1

Take one angle as 2x and the other as x because then their ratio would be 2:1. And when you add them you get 90. So you want to find x,
2x+x=90
3x=90
x=30
One angle is 2*30 or 60 degrees, and the other is 1*30 or 30 degrees. 
And it makes sense. 60:30 is a 2:1 ratio if you divide both sides by 30 and they both add to 90. 
Hope this helps. 

Aand b are correctly balanced reactions. they have the same number of atoms in both sides of the reaction