Answer: The number of letters that must be engraved for the costs to be the same is 125.
Step-by-step explanation:
Since we have given that
[tex]0.4e+10+15=0.20e+5+25[/tex]
We need to find the number of letters.
[tex]0.4e+25=0.2e+30\\\\0.4e-0.2e=30-25\\\\0.2e=25\\\\e=\dfrac{25}{0.2}\\\\e=125[/tex]
Hence, the number of letters that must be engraved for the costs to be the same is 125.