Question 269702
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The time it takes to do homework includes a fixed amount of time to prepare plus a constant amount of time per problem. 
If a student can do 5 homework problems in 40 minutes, and 10 problems in 70 minutes, how many minutes will 25 problems 
take?
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        Calculations in the post by @mananth are incorrect.

        I came to bring a correct solution.



Let the fixed time taken be x
The time to solve a problem be y
x+5y=40------------1
x+10y=70-----------2
subtract equation 2 from 1
(x+5y) - (x+10y) = 40-70
x+5y -x-10y = -30
-5y = -30
y = -30/-5
y = 6. Time taken to solve each problem is 6 minutes
x + 5y = 40 substitute the value of y in the equation
x + 30 = 40
x = 40-30 = 10 minutes will be the fixed time
25 problems will take (10 minutes + 6 *25)
= 160 minutes or 2 hours and 40 minutes.    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<U>ANSWER</U>