How to calculate the abundance of an isotope with only the ram and two isotopes

How to work out how many atoms of iridium 193 naturally occurs in 100 atoms of iridium
Could someone show me how I would work this out step by step please :-)

the atomic mass of Ir = 192.2g/mole or amu
there are 2 isotopes of Ir, 192 and 193

we know that the weighted average mass of 192Ir and 193Ir = the atomic mass of Ir
we also know that the percentage of each isotope must = 100%

this requires 2 equations
let X = amount of 192Ir and Y = amount of 193Ir

X x 192 + Y x 193 = 192.2
X + Y = 1 (100% = 1 in numerical form)

solving for X we get X = 1 - Y.
we plug this into the first equation to determine the amount of Y present
(1 - Y) + 193Y = 192.2
1 + 192Y = 192.2
192Y = 191.2
y = 0.996 or 99.6% Ir = 193Ir
therefore, x = 0.004 or 0.4% 192Ir

so, 100atoms x 0.996 = 99.6 atoms of 193Ir