M = Number of unread magazines
R= Magazines already read
we need to find number of unread magazines (M) after reading 7 magazines (i.e. R = 7)
Unread magazines + magazines already read = Total number of new magazines
M + R = 18
Subtracting R from both sides
M + R - R = 18 - R
M = 18 - R ..............required equation.
substituting R= 7
M = 18 - 7
M = 9
So there will be 9 magazines left to read.
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