First, we need to simplify the fraction:

- 1 3/4 = 7/4

Now we can solve the expression:

2.5 - 7/4 : -3/4

To divide by a fraction, we can multiply by its reciprocal:

2.5 - 7/4 x -4/3

Simplifying the fraction:

2.5 - 7/3

To subtract fractions, we need a common denominator:

2.5 x 3/3 - 7/3

7.5/3 - 7/3

Simplifying:

0.5

Therefore, 40% : 2.5 - 1 3/4 : -3/4 = 0.5.