0.15x + 0.10(250 - x) = 28.75

0.25x + 0.10(250 - x)  =  28.75

Use the distributive property:

0.25x + 25.0 - 0.10x  =  28.75

Combine the x-terms:

0.15x + 25.0  =  28.75

Subtract 25.0 from both sides:

0.15x  =  3.75

Divide both sides by 0.15:

x  =  25