Choose a random number for a and b suppose A=10 and B=5

A can do 1/10 work in 1 min

B can do 1/5 work in 1 min

There is 1 work to do. Since A comes in and does 1/10 work by working alone for 1 min

The remaining work is 10/10-1/10=9/10

If A and B work together they can do 15/50 work in 1 min so they will take 50/15 minutes to complete the total work

However they only have to do 9/10 of the total work so they can complete the total work in 50/15* 9/10 = 3 mins

total time taken 1 min for A and 3 min for A+B = 4 minutes

Replace the value of A=10 and B=5 in the answer choices option B gives 4

This process can take more than 2 minutes but it is just another way of solving the question

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This is my response to the question and may be incorrect. Feel free to rectify any mistakes