Explanation:
Let +a+ = liters of 40% solution needed
Let +b+ = liters of 25% solution needed
+.4a+ = liters of HN03 in 40% solution
+.25b+ = liters of HN03 in 25% solution
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(1) +%28+.4a+%2B+.25b+%29+%2F+57+=+.26+
(2) +a+%2B+b+=+57+
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(1) +.4a+%2B+.25b+=+.26%2A57+
(1) +.4a+%2B+.25b+=+14.82+
(1) +40a+%2B+25b+=+1482+
Multiply both sides of (2) by +25+
and subtract (2) from (1)
(1) +40a+%2B+25b+=+1482+
(2) +-25a+-+25b+=+-1425+
+15a+=+57+
+a+=+3.8+
and, since
(2) +a+%2B+b+=+57+
(2) +3.8+%2B+b+=+57+
(2) +b+=+53.2+
3.8 liters of 40% solution are needed
53.2 liters of 25% solution are needed
check:
(1) +%28+.4a+%2B+.25b+%29+%2F+57+=+.26+
(1) +%28+.4%2A3.8+%2B+.25%2A53.2+%29+%2F+57+=+.26+
(1) +%28+1.52+%2B+13.3+%29+%2F+57+=+.26+
(1) +14.82+=+.26%2A57+
(1) +14.82+=+14.82+
think you should got the answer