Answer: The expression would be 10x+5
x is some unknown number
10x or 10*x represents ten times that unknown number
"sum of 10 times a number and 5" means we add 5 to the 10x found earlier, so we end up with 10x+5
Optionally you can factor out the GCF 5 to get 5(2x+1). Likely your teacher will want you to keep it as 10x+5.