Question: still have problems

still have problems,
1.
restart:
f := proc (x) options operator, arrow; piecewise(0 <= x and x < 4, 50, 4 <= x and x < 5, 0, 5 <= x, f(x-5)) end proc;

plot(proc (t) options operator, arrow; evalf(Int(f, 0 .. t, epsilon = 0.1e-3, digits = 15)) end proc, 0 .. 20);

in the above figure,there is a discont point in the interval [16..18],but in fact,it's a continuous curve.

2.
how to use fsolve deal with two periodic functions?
for example,
g := proc (x) options operator, arrow; piecewise(0 <= x and x < 10/3, 60, 10/3 <= x and x < 13/3, 0, 13/3 <= x, g(x-13/3)) end proc;

fsolve((proc (t) options operator, arrow; evalf(Int(g, 0 .. t, epsilon = 0.1e-9, digits = 15)) end proc)-(proc (t) options operator, arrow; evalf(Int(f, 0 .. t, epsilon = 0.1e-9, digits = 15)) end proc)-500,0..100);

looking for help,thank you.