cannot evaluate symbolic expression numerically

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I'm probably doing something wrong on a fundamental level, so: sorry for my ignorance. Yet i'd very much appreciate any suggestions, how to make this work:

I have me a function f, defined in somewhat lenghty way:

var('a,b,c,d')

i=matrix(SR,3,3, [1,0,0, 0,1,0, 0,0,1])

e_1=matrix(SR,3,3, [1,0,-1, 0,0,0, -1,0,1])
e_2=matrix(SR,3,3, [0,1,-1, 0,0,0, 0,-1,1])
e_3=matrix(SR,3,3, [0,0,0, 1,0,-1, -1,0,1])
e_4=matrix(SR,3,3, [0,0,0, 0,1,-1, 0,-1,1])

M=matrix(SR,3,3, [a,b,1-a-b, c,d,1-c-d, 1-a-c, 1-b-d,a+b+c+d-1])

M_1=e_1*M+M*e_1
M_2=e_2*M+M*e_2
M_3=e_3*M+M*e_3
M_4=e_4*M+M*e_4
A=matrix(SR,4,4, [ M_1[0,0], M_1[0,1],M_1[1,0],M_1[1,1], M_2[0,0], M_2[0,1],M_2[1,0],M_2[1,1], M_3[0,0], M_3[0,1],M_3[1,0],M_3[1,1], M_4[0,0], M_4[0,1],M_4[1,0],M_4[1,1] ])

f(a,b,c,d)=A.determinant()

And then I want a solution to f==0, after giving some random arguments to it

var('k,l,m,n,t,y')

for i in range(5):
    k=random(); n=random()
    if k>n: y=k
    else: y=n
    l=random()*(1-y)
    m=random()*(1-y)
    if k+l+m+n>1:         
        sols=solve([f(k*t+1-t,l*t,m*t,n*t+1-t)==0], t);
        sols[1]

A wild string of numbers pops out. But how do i get to see how much is it? Changing to sols[1].n() gives "TypeError: cannot evaluate symbolic expression numerically".

Thanks in advance!