一个水箱里装有 $6\ell$ 的水。下图显示了每天结束时水箱中剩余的水量。第 2 天和第 4 天一共使用了多少水？ 答：___ $\ell$

1. Determine the scale of the vertical axis

There are $10$ small grid squares between $0$ and $2$, so each small square represents $2 \div 10 = 0.2\ell$.

2. Read the amount of water left at the end of each relevant day

From the graph:

• End of Day 1: $5.0\ell$
• End of Day 2: $4.2\ell$
• End of Day 3: $2.8\ell$
• End of Day 4: $1.2\ell$

3. Calculate the amount of water used on Day 2 and Day 4

Water used on Day 2 = Amount left at end of Day 1 - Amount left at end of Day 2
$= 5.0 - 4.2 = 0.8\ell$

Water used on Day 4 = Amount left at end of Day 3 - Amount left at end of Day 4
$= 2.8 - 1.2 = 1.6\ell$

4. Calculate the total amount of water used on Day 2 and Day 4

Total water used $= 0.8 + 1.6 = 2.4\ell$