A number consists of 3 digits whose sum is 10. The middle digit is equal to the sum of the other two and the number will be increased by 99 if its digits are reversed. The number is:

Let the digits be x, y, z. We know x+y+z=10 and y=x+z. Substituting y in the first equation gives x+(x+z)+z=10, so 2x+2z=10, meaning x+z=5, which means y=5. When reversed, the number increases by 99, so the middle digit must be 5 (100-1=99 works only with middle digit 5). The only option with middle digit 5 that satisfies the conditions is 253.