Answer:
5 months
Step-by-step explanation:
You had a typo and said that Phil weighed both amounts so I'm assuming that the second amount is Bill's weight
We can use LCM for this
So lets make a list for both of them
Phil:120, 130, 140, 150, 160, 170, 180, 190, 200
Bill: 150, 154, 158, 162, 166, 170, 174
As you can see here, after 5 months, they will have the same weight.
We can check our work to.
lets do 120+(10*5)=170
and 150+(4*5)=170
I hope this makes sense