If there are 128 neutrons in pb-210, how many neutrons are found in the nucleus of pb-206?

Pb-210 and Pb-206 are isotopes of the Lead atom. Isotopes are atoms of the same element that have same number of protons, but differ in the number of neutrons. The numbers 210 and 206 represent the mass numbers or the sum of the protons and neutrons in an atom's element. 

For Pb-210,

210 = protons + neutrons
210 = protons + 128
protons = 82

Therefore, for Pb-206,

206 = protons + neutrons
206 = 82 + neutrons
neutrons = 124

Therefore, the number of neutrons in Pb-206 atom is 124. Actually, you don't need to know the number of neutrons in Pb-128 because the number of protons of Pb is known already. It has an atomic number of 82.