Mr. "Iamthe Best" needs your help !!! How to delete a set of rows

Hello,

I have a set of rows from a total 20000 rows, which have some strings.
The entire worksheet consists of five colums of numbers. I am trying to delete these rows by using some formula that could be dragged down a column. Lets say I have this :

123 1444 155 5151
121 1212 141 1515
string
123 1444 155 5151
121 1212 141 1515
string
123 1444 155 5151
121 1212 141 1515

So, I just want to delete the rows with strings by dragging a formula on the last column that does exactly that. Could anyone show me how to do this by any method whasoever. Thanks.