Old version of 4D Expression

I use an old version (v6.0.6) of 4th Dimension. I need an expression to change the last 2 digits of an integer to 50. "change string" doesn't work. What will?

Explaination: Our state now requires the state withholding to be determined by using a "midrange number". IOW the taxable income must be converted rounded to the nearest 50, i.e. both 2013 and 2093 have to be converted to 2050, and 10223 and 10288 have to be converted to 10250. Change string() won't work. What will?