Mathematical Reasoning
Ling can wash a van in 24 minutes working alone.
Question 1 · Multiple choice
Question
Ling can wash a van in 24 minutes working alone.
Mo can wash the same van in 40 minutes working alone.
They start together at 8:40 am and work without stopping until the van is finished.
At what time do they finish?
Options
- A.8:52 am
- B.8:55 am
- C.9:00 am
- D.9:04 am
- E.9:10 am
Correct answer
Explanation
Step 1: Find each person's work rate
Rate = fraction of the job done per minute.
- Ling: 1 van in 24 minutes → 1/24 van per minute
- Mo: 1 van in 40 minutes → 1/40 van per minute
Step 2: Add their rates (combined rate)
1/24 + 1/40
Find a common denominator. LCM of 24 and 40 = 120.
= 5/120 + 3/120 = 8/120 = 1/15 van per minute
Step 3: Find the total time
If together they complete 1/15 of the van each minute, they finish the whole van in:
1 ÷ (1/15) = 15 minutes
Step 4: Find the finish time
Start: 8:40 am
8:40 am + 15 minutes = 8:55 am
Answer: They finish at 8:55 am