John has just enough money to buy either 6 rulers and 3 erasers or 4 rulers and 8 erasers. He spends all the money on erasers, how many erasers can he buy? Ans : ___

1. Equate the two scenarios to find the relationship between the cost of a ruler and an eraser.

Let $R$ be the cost of a ruler and $E$ be the cost of an eraser.
$6R + 3E = 4R + 8E$
$6R - 4R = 8E - 3E$
$2R = 5E$

2. Express the total amount of money entirely in terms of erasers.

Total money $= 6R + 3E$
Since $2R = 5E$, we have $6R = 3 \times 5E = 15E$.
Total money $= 15E + 3E = 18E$
He can buy $18$ erasers.