Uh can someone explain how they got the answer too

Carson has only $20 bills and $10 bill in her wallet. the total value of the bills is $50. she has 1 more $20 bill than $10 bills. how many each kind of bill does carson have? a. one $20 and two $10 bills b. two $20 bills and one $10 bill c. one $20 bills and three $10 bills d. two $20 bills and two $10 bills

What number should be added to both sides of the equation to complete this square? x^2-10x=7

For each proportion find the unknown number x 1/5/8=×/15