Let, number of solid color gift sold is x and printed gift wrap is y.
Also, number of printed paper sold , 325.
Expressing, it in mathematical equation :
y = 325 ....... 1)
Also, The band needs to earn $2,500 for an upcoming competition.
So ,
3.5x + 5y = 2500 ....... 2)
Solving equation 1 and 2 :
3.5x + 5(325) = 2500
3.5x = 2500 - 1625
3.5x = 875
x = 250
Therefore, number of solid paper they must sell is 250.
Hence, this is the required solution.