I have this problem I can't solve.
After having been shaped by heat, a piece of metal with temperature of 1500 degrees Celsius must be dropped into a tank of water to cool. The tank of water is kept at a temperature of 35 degrees Celsius after 10 min in the cooling tank, the metals temperature temperature drops to 750 degree Celsius. The rate of decrease in the metals temperature is said to be negligible when the metal reaches 75 degree Celsius, so the metal is removed from the tank when it reaches this temperature.
The change in the temperature of the piece of metal follows Newton law of cooling which states that the difference in temperature between an object and its surroundings decreases exponentially according to the relationship T-Ts = (T0 - Ts) ^ekt, where T is the object temperature at time t, Ts is the temperature of surroundings, T0 is the initial temperature of the object. To the nearest hundredth, what is the negligible rate of temperature decrease for this particular metal?

Question

I have this problem I can't solve. 
After having been shaped by heat, a piece of metal with temperature of 1500 degrees Celsius must be dropped into a tank of water to cool. The tank of water is kept at a temperature of 35 degrees Celsius after 10 min in the cooling tank, the metals temperature temperature drops to 750 degree Celsius. The rate of decrease in the metals temperature is said to be negligible when the metal reaches 75 degree Celsius, so the metal is removed from the tank when it reaches this temperature. 
The change in the temperature of the piece of metal follows Newton law of cooling which states that the difference in temperature between an object and its surroundings  decreases exponentially according to the relationship T-Ts = (T0 - Ts) ^ekt, where T is the object temperature at time t, Ts is the temperature of surroundings, T0 is the initial temperature of the object. To the nearest hundredth, what is the negligible rate of temperature decrease for this particular metal?