Blank cells if other is blank too

[FloDaCo]

Hello ,

I want cell H15 to be blank if G15 is blank too. However, if G15 has a number in it, H15 should be the double of the number in G15. The same should count for the entire colomn: if G16 is blank so should H16 but if there is a number in G16, H16 should have double that number et cetera.

I tried the following formulas (and some more) but none work:
<style type="text/css">p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px 'Lucida Grande'}span.s1 {color: #0057d6}</style>
=if(G15="","","G15*2")
<style type="text/css">p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px 'Lucida Grande'}span.s1 {color: #0057d6}</style>
=if(G15="ISBLANK","ISBLANK","G15*2")

I hope someone can help. Thank you in advance.

 

[123rickfear]

=if(isblank(G15),"",G15*2)

 

[godsaaint]

You almost had it with the formula, just have to delete the last "":

=if(G15="","",G15*2)