Answer:
They will need 25 boy costumes and 35 girls costumes.
Explanation:
There are x boys.
There are x + 10 girls.
Altogether, there are x + x + 10 students.
Altogether, there are 60 students, so
x + x + 10 = 60
2x + 10 = 60
2x + 50
x = 25 (boys)
x + 10 = 35 (girls)