To do questions in this form ,you should clearly know the basis.It can be achieved by doing a lot of questions and a student like you can surely achieve that but by hardwork.
To do such problems you should know the constant values such as calories at 0 degree, 100 degree etc.You. can find those values in books or simply you can find it on internet.
Here,Calories required to convert one gram of water at 100 degree celcius in to steam is 540 Calorie(ofcourse it is a constant value)
So here we just need to multiply the constant with number of grams of water.
So,Calories reqd = 540 × 5 =2700 calories.