Sucrose has a denisty of 1.58g/ml. What is the mass of 87.5 ml of sucrose

8.5

The units have to be right. If you have (B g/ml) and (C g), arrange the ratios such that the units cancel. In this case: (B g/ml) * ( 1/(C g)). The units of the answer would be in units of grams.

Conversions of length can be done the same way:
Suppose 2.54 cm = 1"; 12" = 1' and you were asked "What is the number of feet in 30 cm"?

You know you have 30'
You multiply 30' x 12"/1' and I have the result in inches because units cancel.
You then take (30' x 12"/1') and multiply by 2.54 cm/1" and I get the result in cm.
feet * inches/feet * cm/inches = cm