Numeric Entry equal to a specific value

Hey, all. I am working on a numeric entry activity and I am wondering if it is possible to set a numeric entry equal to a value of 150 when the user clicks on that numeric entry field. I created an image of the number 150 highlighted in blue, and put the numeric entry on top of that. It currently functions so that when the user clicks on the numeric entry, the image with the highlighted blue 150 becomes hidden, so it looks like it disappeared from the text box. However, I want it to look like only the highlight disappears and the number 150 remains. So I created a trigger to set the numeric entry to "150" when the user clicks in the numeric entry field. However, it is not working. Does anyone know how to make this work? Thanks!