The distribution of dog weight is normally distributed with a mean of 18 pounds and a standard deviation of 3 pounds. Therefore, 84% of dogs weigh below how many pounds?

Here you have to work with z-score table.

Formula: z = (x-μ) / σ, where x is the # of dogs, μ = mean and σ = standard deviation.

You look up in the score table for which value, z corresponds to 0.84 and you will find that z = 0.7995 for a probability of 84%

0.7995 = (x-18) / 3 → x = 20.39 or 84% weighs under 20.39 pounds