John has two colored varieties of water bottles: white water bottles and black water bottles. The total number of water bottles is 20. He sold a white water bottles for 1 and a black water bottles for 2 and earned a sum of 35. Find number of black water bottles he has.

the number of black water bottles he has is 15

Step-by-step explanation:

Let x represent the number of white water bottles that he has.

Let y represent the number of black water bottles that he has.

The total number of water bottles is 20. It means that

x + y = 20

He sold a white water bottles for 1 and a black water bottles for 2 and earned a sum of 35. It means that

x + 2y = 35- - - - - - - - 1

Substituting x = 20 - y into equation 1, it becomes

20 - y + 2y = 35

- y + 2y = 35 - 20

y = 15

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