A can complete a work in 5 days and B can complete the same work in 10 days. If A and B work alternately, starting with A, in how many days will they complete the work?

$6\frac{1}{2}$ days

A = 5 days, B = 10 days,

⇒ As A and B works alternatively, starting with A,

 1st day = 2 units

 2nd day = 1 unit

Every 2 days = 3 units of work,

Therefore, in 6 days, 9 units of work will be completed

Hence, in \( { 6}_{2 }^{1 } \) days, 10 units of work will be completed.