Put together, Dulcina and Tremaine have 129 total matchbooks. Tremaine's collection has 39 fewer matchbooks in it than Dulcina's collection. How many matchbooks are in Tremaine's collection? How many matchbooks are in Dulcina's collection?

Let the Dulcina's collection be 'x'

Let the Tremaine collection be 'x-39'

x + x - 39 =129

2x = 129 +39

2x = 168

x = 168/2

x = 84

Dulcina's collection = x = 84

Tremaine's collection = x - 39 = 84 - 39 = 45

step-by-step explanation:

12: 00 p.m. to 7: 00 p.m. is a total of 7 hours.

4: 00 p.m. to 7: 00 p.m. is a total of 3 hours.

this means that from 4: 00 p.m to 7: 00 p.m. this represents 3/7 of what was lost from 12: 00 p.m. to 7: 00 p.m.

after knowing this, after doing 11,600 - 6,800 = 4,800

4,800 represents 3/7 of what was lost for 7 hours.

we then divide 4,800 by 3 which gets 1,600.

because we know that 1,600 represents 1/7 of how much water was lost, we just have to multiply by 7, which gets 11,200.   after adding 11,200 to how much water was left at 7: 00 p.m. which was 6,800. 11,200 + 6,800 = 18,000

this means that the pool originally had 18,000 gallons in the pool.

you can also check your work by doing 18,000 - 4(1600), which becomes           1800 - 6400 which gets you 11,600.