Let x = # chapters you have left to read
And you also know that the amount you have left to read is the total number of chapters minus the amount you already read.
The total number of chapters is 8.
The amount you already read is 3/8 * 8 = 3.
Thus:
x = 8 - (3/8 * 8) = 8 - 3 = 5.
You have 5 chapters left to read.
