Question: differential equation systems procedure

i have a systems of differential equation:

J[n]=2*A[n]

S[n+1]=J[n]

A[n+1]=S[n] + 0.5*A[n]

with initial condition A[0]=10, J[0]=30, S[0]=5

i have a had time writing a procedure for this systems. help please

do i need a for command for this procedure

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