Intersecting numbers

For lack of a better description I am looking for a way to calculate how long it will take 2 increasing numbers to intersect each other or overtake.

For example

A1 = 100 and increases by 1 daily
B1 = 1 and increases by 3 daily

Day 1 - A1 = 100; B1 = 1
Day 2 - A1 = 101; B1 = 4
Day 3 - A1 = 102; B1 = 7
Day 4 - A1 = 103; B1 = 10....

I am looking for how many "days" it will take for B1 >= A1 to occur.

The only work around I have was to make a table where A1 was 100 and A2 was "A1+1"... B1 was 1 and B2 was "B1+3". Added some if statements so when column B was >= column A they stopped increasing then just did a countif function to find out how many "days" occurred.

Does anyone have a more simple solution?