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.

15 black water bottles

Step-by-step explanation:

let w represent white water bottles, and let b represent black water bottles.

set up a system of equations:

w+b=20

w+2b=35

(the top equation shows that the number of white water bottles he sold and the number of black water bottles he sold equals to 20, since it says that the total number of water bottles is 20. the bottom equation shows that white water bottles cost $1, and black water bottles cost $2, and that the total amount of money he got from selling them is $35).

in the top equation, isolate w:

w+b=20

w=20-b

now we do substitution. in the bottom equation, substitute 20-b for w:

w+2b=35

20-b+2b=35

then, solve:

b+20=35

b=15

this means that he sold 15 black water bottles.

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step-by-step explanation: