for 3). if 10% of abc is 5 then it means that abc = 50.
Hence, a,b and c must all be divisible by 50 (assuming whole numbers only)
a+b cant be 1 because we are talking only positive integers (see question).
if a+b equals 3, the either a=1 and b=2 or a=2 and b=1. If this was the case then ab would equal 2. Hence c could be 25. This is a possibility
if a+b=5, then combinations are (1,4), (2,3), (3,2), (4,1). If ab was 4, then c cant be a whole number so we can discard. if ab was 6, then c cant be a whole number so we can discard.
if a+b=6 the combinations are (1,5), (2,4), (3,3), (4,2), (5,1). if ab=5, then c could be 10. Hence a possibility. if ab=8, the not divisible by 50. if ab=9 then not divisible.
if a+b=25, then ab could numerous combinations. Will let you work this out.
Hence 3 and 6 are possibilities