Mass of sucrose solutions
If 10ml of 40% sucrose is 15.7 grams, then the mass of 150 ml of 40% sucrose is 

$$ 15 \times 15.7 grams$$

You can not determine the mass of 150 ml of 20% sucrose from the information given.

You could calculate from this sucrose solution density table:
http://homepages.gac.edu/~cellab/chpts/chpt3/table3-2.html

Note that the table shows the premise "I know is the mass of 10 ml of 40% sucrose, which is about 15.7 grams" to be incorrect.