The addition property of equality justifies this. You could also say that she simply subtracted 7, and then it would be the subtraction property of equality. This is the case because if you add or subtract (or multiply or divide) the same number on both sides of an equation, the equation will still be the same.
Hope it helps!
For cylinders to be similar, the ratio between their height and radius must be equal.Our cylinder's ratio is :

The only cylinder with same ratio is the one at the choice B.
The number of cats is 2.5 times the number of dogs.