Normally, I would set this up by letting x be the number of 20 cent coins and y be the number of 50 cent coins. But you titled this "linear equation involving one unknown" so let x be the number of 20 cent coins. Since "the number of 20 cent coins is 23 more than the number of 50 cent coins", the number of 50 cent coins is x- 23. The value of all the coins is 20x+ 50(x- 23) and that is 26.20$. Assuming that 1.00$ is "100 cents" (what currency is this?), the equation is 20x+ 50(x- 23)= 2620.