It takes them 1 7/8 hrs to complete the job together.
How:
Fred = 3 hrs
Jason = 5 hrs
1/3x + 1/5x = 1/x
3x x 5x = 15 —> 3x/15 + 5x/15 = 15/15
3x + 5x = 8x —> 8x/15 = 1.875 = 1 7/8 hrs
1 kilogram = 1000 grams
100g • 10 = 1000g
Isaiah would need to put 10 100 gram weights on the scale in order for it to equal 1 kilogram.
31 divided by 9548 is 0.00324675