One thousand Ca 2+ channels open in the plasma membrane of a cell that is 1000 m

One thousand Ca²⁺ channels open in the plasma membrane of a cell that is 1000 μm³ in size and has a cytosolic Ca²⁺ concentration of 100 nM. For how long would the channels need to stay open in order for the cytosolic Ca²⁺ concentration to rise to 5 μM? There is virtually unlimited Ca²⁺ available in the outside medium (the extracellular Ca²⁺ concentration in which most animal cells live is a few millimolar), and each channel passes 10⁶ Ca²⁺ ions per second.