Hey there! So since the water is at 100 deg C, we know that its undergoing vaporization. Anytime there is a phase change, we need to use this equation: q=m x deltaH. deltaH of vaporization should be given in the problem.
When we found q, we found the total heat absorbed. Next use the rate given to find how long it would take to absorb all of the heat, which is the same as all the water turning into steam. Let us know if you have any further questions on this.