assume the amount of money they both deposited in october is m.
in november, greg deposited 120$, so the total amount in his account became m+120
thomas increased his account by 20%, this meams that the amount of money in his account became: m+0.2m
since both values are still equal, we will equate the two equations and solve for m:
m+120 = m+0.2m
0.2m = 120
m = 600$