The price of salt goes up by 9 cents. The old price goes up by four-fifth of new prices. Find old and new price.
n = o + 9
o + (4/5) n = n
so
n - 9 + 4 n/5 = n
4 n /5 = 9
4 n = 45
n = 45/4
o = 45/4 - 36/4 = 9/4
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now check
n = o + 9 = 9/4+36/4 = 45/4 yes
o + (4/5) n = n
9/4 + (4/5)(45/4) = n ?
5*9/4 = n ?
45/4 = n yes