Translating that problem with x being our variable you get 3x + 5x-11 = 15. Combine like terms to get 8x-11 = 15. Add 11 to both sides to get 8x = 26. Divide by 8 on both sides to get x = 26/8. This can be reduced to 13/4 or 3 and 1/4.
Now we plug in. Our first number was represented by 3x. So 3*(13/4) = 39/4 or 9 and 3/4. The second number was represented by 5x-11. 5*(13/4) = 65/4 or 16 and 1/4. 16 and 1/4 minus 11 is 5 and 1/4.
To check our answer 9 + 5 = 14. 3/4 + 1/4 = 4/4 or 1. 14+1 = 15. Another way to check is adding (39+21)/4.
So your complete sentence should read: The two numbers whose sum equal 15 in this equation are 5 and 1/4 (or 21/4) and 9 and 3/4 (or 39/4).
