It doesn't require any calculations, really, just logic. Look, 4n = 4n wil be equal because they are the same numbers multiplied by the same variable - no matter the variable's value. However, if you subtract both values by different numbers, you won't get the same number:
Let t = number of terriers 1 x t = number of total terriers 5 x t = number of yellow labs 1t + 5t = 6t 6t = total number of dogs in the park 6t = 18 t = 18/6 t = 3 there are 3 terriers
