Suppose a rock sample at the time of its formation 3.6 × 10^9 years ago contained N0[87Rb] rubidium atoms 87 (87Rb), N0[87Sr] strontium atoms 87 (87Sr) and N0[86Sr] strontium atoms 86 (86Sr). From the relative abundance of the three species at the height of the rock formation yielded the following ratios: N0 [87Rb] / N0 [86Sr] = 0.2 and N0 [87Sr] / N0 [86Sr] = 0.7. The 87Rb decays into 87Sr with a half-life of 48.8 × 109 years. Turn the 86Sr and 87Sr are stable. Determine the percentage of 87Rb atoms that have survived to this day and Determine the ratio N [87Rb] / N [86Sr] that currently exists in this sample of rock .. Can someone explain me how this thing of the ratio works? I don't understand what they want in the question?