A ratio isotope has a half life 10 days. If today there is 125 g of it left, what was its weight 40 days earlier?

Question

$$\begin {array} {1 1} (a)\;600\;g & \quad  (b)\;1000\;g \\  (c)\;1250\;g & \quad (d)\;2000\;g  \end {array}$$