If a dna sample contains 10 thymine what would be the percentage of guanine

Do you mean 10 percent thymine? If so, the guanine will have the 40% of the DNA. This is because thymine will always pair with adenine, thus will also have 10 percent. Guanine will always pair with cytosine which will also get 40%. The total percentage of 100 % is divided to thymine and adenine of 20 percent and guanine+cytosine of 80 percent.