To solve the equation 0.8x + 3.5x - 0.75 = 1 2/5, we first need to simplify the right side.

1 2/5 is equal to 7/5, so the equation becomes:

0.8x + 3.5x - 0.75 = 7/5

Now, combine the x terms:

4.3x - 0.75 = 7/5

Next, add 0.75 to both sides of the equation:

4.3x = 7/5 + 0.75
4.3x = 7/5 + 3/4
4.3x = 28/20 + 15/20
4.3x = 43/20

Now, divide by 4.3 to solve for x:

x = (43/20) / 4.3
x = (43/20) * (1/4.3)
x = 43/86
x = 0.5

Therefore, the solution to the equation 0.8x + 3.5x - 0.75 = 1 2/5 is x = 0.5.