Piya, Siya and Diya alone can complete a piece of work in 'a' days, 'b' days and 'a + b' days respectively. Piya and Siya together can complete the same work in 6\(\frac{2}{3}\) days. It takes 7.5 days for Siya and Diya together to complete the same task. If Piya and Diya together can complete it in 12 days, find the value of 'a - b'.
1
a/2
2
2b
3
b/2
4
2a
5
None of these