Write 2 equations, multiply amount bought per month by number of months and add to the starting values :
Marko = 45 + 4M
Tamara = 61 + 2M
Set the equations equal to each other and solve for number of months:
45 + 4m = 61 + 2m
Subtract 45 from both sides:
4m = 16 + 2m
Subtract 2m from both sides:
2m = 16
Divide both sides by 2:
m = 8
Answer: 8 months