Explanation:
step 1. let Mason = m and Tatum = t
step 2. m = 3x + 4 ($3/month and one time $4)
step 3. t = 1x + 20 ($1/month and one time $20)
step 4. 3x + 4 = x + 20 (set equations equal)
step 5. 2x = 16 (collect like terms)
step 6. x= 8 (answer: 8 months)