Two objects made of the same material are heated to 60 oC and 90 oC. According to

Newton's law of cooling, which would cool faster and why?

Assuming that there is in a vacuum, the two object will cool at the same rate, because the objects are made of the same material they will have the same cooling rate, assuming the surrounding temperature is the same.

The newton law of cooling is:

dQ/dt = - h*a (To - Ta)

Where h and a are constants, To is the temperature of the object and Ta is the temperature of the ambient.

this means that initially if in both cases Ta is the same, the object with higher temperature cools faster.

Now, obviously, if we want to know which object cools faster to a given temperature, for example, 30°C, the object with 60°C will reach that temperature in less time than the other one. (this is because the hot object cools faster at the beginning, but as it starts to lose temperature, the rate at which it colls starts to decrease)