to a) I assume that the "2,5 saltsolution" is actually 2.5% saltsolution. If so:

Calculate the total amount of salt:

50 g + 2.5% * 500 g = 50 g + 12.5 g = 62.5 g

Calculate the total mass:

800 g + 50 g + 500 g = 1350 g

Therefore you have the ratio

to b) For convenience I use grams:

Calculate the total amount of detergent in 3 g of the 20%-solution:

of detergent.

The total mass of the thinned solution is 53 g (You added 50 g to the original 3 g of concentrated detergent)

Therefore you have the ratio

to c) Let V denote the unknown volume of the 45%-acid.

Calculate the total amount of acid:

. Collect all terms with V at the LHS, all constants at the RHS: