relative atomic mass. chemistry help!?

[NF123]

how do you do this question?

Hydrogen has three isotopes, protium with no neutrons, deuterium with one neutron and tritium with two neutrons. In a sample trapped in a meteorite, they are found to occur in the ratio 10: 1: 1.

What is the relative atomic mass of the hydrogen in the sample?

i know what the answer is. it's 1.25 but i want to know how you get that answer..

please help!

 

[lavalamp3773]

10 * 1 + 1 * 2 + 1 * 3 = 15

15 / 12 = 1.25

Though using more accurate data for the atomic masses, the relative atomic mass comes out to be 1.259.