Hint:
Their usual meeting time = 5/2.5 = 2 hours
Let's assume that they met for 2 hours "x" no. of times.
So,
2x + 5 = 21
2x= 16
x = 8
This means that they met for "2 hours i.e., usual time", 8 times.
So, 8 usual meetings + 1 meeting of five hours = 9 meetings in total
