Mass number and proton count jointly set neutron count. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Fourteen and six leave eight neutrons
Mass number 14 minus proton count 6 leaves neutron count 8.
N=A−Z=14−6=8
Twelve and five leave seven neutrons
Mass number 12 minus proton count 5 leaves neutron count 7.
N=A−Z=12−5=7
Sixteen and eight leave eight neutrons
Mass number 16 minus proton count 8 leaves neutron count 8.