Question 1051070
{{{C[1]=2+0.50N}}}, {{{N>=9}}}
{{{C[2]=7}}}
So find {{{N}}},
{{{C[1]=C[2]}}}

{{{2+0.5N=7}}}
{{{0.5N=5}}}
{{{N=10}}}
At 10 checks (plus the original 8 free checks), the costs would be the same.
So to keep it less than the second account, you could write at most 17 checks.