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Ben and Greg go to the movies and purchase snacks. Ben purchases four cookies and two bottles of water for a total cost of $9.00.
Greg purchases two cookies and two bottles of water for a total cost of $7.00. What is the cost of a bottle of water?
Good style in solving such problems is to notice/(to observe) immediately from the condition that the difference
between two combinations is the cost of 2 cookies, and it is equal to $9 - $7 = $2. So 2 cookies cost $2.
Then subtracting $2 dollars for 2 cookies from the Greg purchase, you got the cost of two bottles - it is equal to $7 - $2 = $5.
Answer. A bottle of water costs $2.50.
This problem is for mental solution, and it would be ideally if you see the solution immediately/instantly as you completed reading its condition.
There is no any need to write and to solve a system of equations.
See the lesson
- Solving mentally word problems on two equations in two unknowns
in this site.
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Such problems are good for using in interviews, if the employer want to check whether the interviewed had special training in Math.
If your solution follows my scheme, the interviewer marks "YES" in his check-list.
If you follow the @josgarithmetic way, the interviewer marks "NO" in the check-list.
In the second case, the interviewed person even doesn't know and doesn't understand for what reason the question/(the problem) was given to him.