factor 100+100rt
⚠ rt is read as the product r*t: for the variable rt, write rt_
The expression: 100 + 100rt contains several variables so we pick r for factoring
Let's factorize in ℝ: 100 + 100rt
We factor out 100, the greatest common divisor of the coefficients: 100 + 100rt = 100·(1 + rt)
Factor found: rt + 1
▶Factored expression:   100·(rt + 1)