This Math problem can be written and solved as a system of 2 linear equations with 2 unknown variables X (grape cost) and Y (orange cost)
(2 1/2)X + (1 1/2)Y = 12.5
X - Y = 1
You can solve this system of linear equations by using online equations solver, which accepts input as fractions, mixed numbers and decimals (http://examn8.com/SystemOfEquations.aspx). The result would be X = 3 1/2 (grape cost) and 2 1/2 (orange cost).
Alternatively, you can convert everything to the decimals and solve it:
2.5X + 1.5Y = 12.5
X-1 = 1
resulting in the same: X = $3.50 and Y = $2.50
Hope this will help.