20 is equal to 5 less than twice a number.

If we have a number represented by n, we have 20 is equal to, so 20=something. That something is "5 less than twice a number". 5 less than something means something-5, and something-5=20. Since 5 is less than twice a number, and twice a number is n+n=2n, we have 2n-5=20
Actually solving the problem, we can start by adding 5 to both sides, resulting in 2n=25. Next, we can divide both sides by 2 to get n=12.5