Let B be the amount Beatrice charges per hour per child
Let x be the number of hours
Let n be the number of children
a)total amount Beatrice charges = T = B*x*n - 10
b)n =4 and T = 100
100 = B*x*4 -40
x = 35/B
c)Let S be the amount she has t o pay to her sister
n = 3 and x = (4+4+5) = 13
so, 13*B*3 -30 -S = 100
S = 39B - 130