X-Therm

John invented a new thermometer. He called it X-Therm. John knows that, $-30^\circ$ X represented the freezing point of water, and $90^\circ$ X represented the boiling point of water.

One day, John measure a water with his thermometer, for his science essay, it showed $60^\circ$ X. The problem is, John has to use the International System of Units (SI) for his essay - which is the Kelvin scale. According to the data above, what is $60^\circ$ X in Kelvin ?

Hint : The Kelvin's freezing point is 273, and the boiling point is 373