convert a number into a whole number block

I would be very grateful for your help.

i don't know how to explain myself, other than i would like a formula which would convert any given number (23) into it's whole number block, ie (20) etc if i type the number 15, it should be converted to (10) and so forth.

please see attached file.

apologies if i have not explained myself properly.

regards

Terence