154853

I expressed the mean of the first three numbers by (20 + x)/3 and the mean of the last four numbers by (33 + x)/4 Since those means are equal, I obtained an equation:

(20 + x)/3 = (33 + x)/4

I then multiplied both sides by 12 and obtained the following:

80 + 4x = 99 + 3x

Then I took away 80 from both sides:

4x = 19 + 3x and then subtracted 3x from both sides and obtained:

x = 19

Therefore, the missing number is 19.

When we plug it in into the original question, the mean of the first 3 numbers is 13 and the mean of the last four numbers is also 13. The means are, therefore, equal which indicates that our answer is correct.