4th order Runge-Kutta Method with multiple equations

dG/dt = -p1*(G-Gb)-XG+(Gmeal/v1), G(0)= G0 dX/dt = -p2*X + p3*(I-Ib), X(0)= 0 dI/dt = -n*(I-Ib)+ (U/(100*v1)) , I(0) = I0

I am provided with values for every variable except G, I, and X. I have figured out how to solve it by hand, but I can't seem to figure out the coding part correctly. Gmeal changes as t changes.

0.96 / min for 120 < t < 140 (all < signs are less than or equal to) 0.72 / min for 360 < t < 390 0.36 / min for 540 < t < 560 1.2 / min for 720 < t <760 0.0 / min for other times

I can provide more details from the problem statement if necessary. I am trying to solve for G, I , and X.

(Also, if anyone can help with part 2: simulate a system for U = 0 and typical intake, for which problem statement variables have been listed above.)