You will look for two equations in two unknowns.
The first equation is Neil (N) plus Lucy's (L) total amount:
N + L = 200    (1)
The 2nd equation comes from the info about Neil spending $50 then having 3 times Lucy's amount:
N - 50 = 3L    (2)
These can be solved for L and N.  For example, I solved (2) for N (N = 3L+50) and then substituted the RHS for N in (1) which gave me   L=$37.50,  then using this value of L in (1) I computed N=$162.50
I've checked these values meet the two conditions, and you should too.