We are given that Tom earns 15 per week and that he already has 21. If "x" is the number of week, then his earnings are given by:

Since the minimum he must save is 120 then the earning must be greater or equal to that, therefore:

To solve for "x" we need to subtract 21 to both sides:

dividing both sides by 15:

Solving the operation:

Therefore, he needs to work for at least 6.6 weeks