1/2a - 4
Add 4 to both sides.
1/2a = 4
Divide both sides by 1/2.
a = 8
2×2
2×2×2×2×2×2×2×2
You can cancel out two 2s from the top and bottom
1
2×2×2×2×2×2
1/ 4×4×4
1/16×4
1/64
Calculate total pages:
(216 pages per book)(13 books) = 2,808 total pages
Now divide by pages per day: Irene reads 18 pages a day
2,808 total pages / 18 pages per day = 156 days to read 13 books