.
This problem is posed / formulated I N C O R R E C T L Y.
- Why ? -
Because there is NEITHER logic NOR functional connection between the first sentence
"A certain concrete mixture contains 6.00% cement and 8.00% sand."
and the second (and the last) sentence/question
"how many kilograms of sand should be combined with 255 kg of cement to make aa batch that is 14.0% cement and 16.0% sand? "
Simply the person who created and formulated the problem, does not know the subject and does not possess terminology.
Naturally, it means that the solutions of both other tutors make no sense.
All this stuff must be redirected to the GARBAGE BOX immediately.
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Meanwhile, in this site there is a HUGE collection of lessons devoted to mixture problems,
and among these lessons there is one containing VERY SIMILAR samples and templates of solved problems
- Advanced mixture problems