solve neatly

7) As a result of the Aykhal Crystal detonution in 1974 (in the former Soviet Union), cesium137 was measured (in 1993) in the soil at a concentration of 2105Bq/kg soil. If the background concentration of 137Cs is 0.5Bq/kg soil, how many more years will it take before the concentration of 137Cs reaches background levels? Assume the decay of radionuclides occurs by a first-order reaction, and the half-life of Cs is 20 years. (half-life means the time it takes to drop 50% of concentration) 20pts