The math club is doing a fundraiser to raise money to attend a math competition. The club was told by their advisor to raise $2000, but they decided to raise $4000. The club is selling "I like π" T-shirts for $10 and "I like π" sweatshirts for $20. Let x equal the number of T-shirts sold and let y equal the number of sweatshirts sold. Write a linear equation to represent raising $4000.