Linear interpolation with lower and upper quartiles

The questions consists of a grouped frequency table that has gaps.
11-20 F:62
21-25 G:88
26-30 F:16
31-35 F:13
36-45 F:11
46-60 F;10
I've got a question asking me to prove the lower quartile is 18.6.
I know that the position of Q1 is in the first class which is 11-20.
However since the first class starts with 11 do i need to change the intervals to 11.5-20.5