For 24, try starting with the girls. Out of the $10$ spots there are for the students to finish pick some for the girls and leave the rest for the boys.
On 26, to choose a team you need to make sure that for each soldier, the soldier's boss (sergeant), or the boss of their boss (officer) is also part of the team. First you need to decide if the team will have (i) $2$ officers OR (ii) $6$ sergeants, and then pick the soldiers. For each of the two cases, how many soldiers do you have available? In how many ways can you choose the officers or sergeants?