we are given
Mark clean his father's office in 30 minutes
so, the amount of work done by Mark in 1 minute is 
younger sister Lynn can clean the office in 40 minutes
so, the amount of work done by younger sister Lynn in 1 minute is 
total work done by them in 1 minute is 
now, we can simplify it
total time taken to complete same work is 
........Answer
Answer:
The answer is s = 1.
Step-by-step explanation:
Opposite sides of a parallelogram are equal so,
3s + 19 = s + 21
3s - s = 21 - 19
2s = 2
s = 1
next,
s + 21 =? 3s + 19
1 + 21 equal to (?) 3(1) + 19
22 = 22
so the answer is correct
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