Answer:
5 weeks
Step-by-step explanation:
The amount of money he has to earn in order to buy the phone is: 400-55 = 345( dollars)
He gets 345 dollars in: 345÷75= 4 weeks and remains 45 dollars
So he needs to work for 1 more week to have enough money to buy the phone
So he needs to work at least 5 weeks