Quite a tricky problem.
Firstly we need to convert it into work/rate problem properly.
Every week the two of them would use up:
1/20+1/8 = 7/40*x worth of detergent per week.
Now we know that the usage decreases by 90 per week.
Which means that the amount of detergent saved is 90*10 = 900 over the course of the time.
How much time? Stem says they use it once after 70 days, or 10 weeks.
Now with a rate of 7/40 per week -> Number of weeks to complete the detergent = 40/7.
There is an addtion -> 10 - 40/7 = 30/7 days.
We have an additional 30/7 days that the detergent lasts because we saved 900grams in total.
Thus 30x/7 = 900
x = 210.
210 grams of detergent is used per week.
Now we know from the first part that 7/40 of work is done per week.
Thus 7x/40 = 210
x = 1200.
1200 grams of detergent in total is being used by them.
kevincan
When they lived alone, Ada used one standard-sized box of Clover laundry detergent every 20 weeks, and Pablo used one standard-sized box every 8 weeks. After they began living together, their total detergent usage decreased by 90 grams per week. If they now use one standard-sized box every 10 weeks, what is the net weight of a standard-sized box of Clover laundry detergent?
(A) 600 grams
(B) 750 grams
(C) 875 grams
(D) 1,000 grams
(E) 1,200 grams