The Atomic Mass!!

[rumaithya]

Hello, I have a problem of getting the answer of this question and I still don't understand it. The question is

According to the Periodic Table, the average atomic mass of a gallium atom is 69.72 amu. Naturally occuring gallium has two nuclides ("isotopes"). If you have a pile with 1,700,000,000,000,000,000 atoms of gallium, how many of them will weigh 69.70 amu?

What I though is that the answer is "1,700,000,000,000,000,000" and I thought it's same as when we say we have a red apple and how many red apples are in 1700000000 red apples. That's sounds strange and probbly wrong i'm not sure [g)] . Do we have to do this -> 1,700,000,000,000,000,000 / 69.72 to get the answer? [:((]

... Any ideas???
Thanks

[GCT]

Find the atomic mass of the two gallium isotopes.

69.72 =[(number of Isotope 1)(atomic mass)+(number of Isotope 2)(atomic mass)] / 1,700,000,000,000,000,000

Solve so that you obtain (number of isotope 1)/(number of isotope 2)

Try to do it yourself first.

The above will be the proportion. For example if the proportion is 1/3 the total percentage of a particular isotope will be either 1/4 or 3/4. Multiply this percentage with 1,700,000,000,000,000,000.

[rumaithya]

Thanks alot for your help [:)]