Answer:
The answer to your question is the first rain lasted 40 hours and the second rain lasted 30 hours.
Step-by-step explanation:
Data
1st 15 ml/h = x
2nd 30 ml/h = y
Total time = 70 h and 1500 ml
1.- Write 2 equations
                First rain + second rain = 70 h
                       x + y = 70    (I)
                15x + 30 y = 1500    (II)
2.- Solve equations by elimination
Multiply first equation by -15
                -15x - 15y  = - 1050
                15x + 30y =  1500
                 0x + 15y = 450
                      y = 450 / 15
                      y = 30 h
                  x + 30 = 70
                 x = 70 - 30
                 x = 40 h
3.- Conclude
The first rain lasted 40 h and the second rain lasted 30 h.
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