So for the first one, the answer would be (b-20)/4=the number of beads per necklace
Because:
You have a total of beads at the start wich is represented by b
You subtract 20 beads from this total, so b-20
You then divide the rest into 4 necklaces, so (b-20)/4
For the second one, the equation is time left=45-6j-2s, and the answer in this case is 17 minutes
Because:
You start with a total of 45 minutes
She does 6 laps, and the time for these laps are represented by j
She does 2 stretches, and the time for these stretches are represented by s
You now have your equation: 45-6j-2s
Since the time required for a lap is 3 minutes and the time for a stretch is 5 minutes, you replace j and s by 3 and 5 respectively.
Giving you 45-6(3)-2(5)= 17 minutes left