Maths problem： how to do, thanks?

Let n be the required number.

Consider
(22 + 24 + n)/16 > (30 + 42 + 21)/24
(46 + n)/16 > 93/24
46 + n > 93/24 × 16
46 + n > 93/3 × 2
46 + n > 31 × 2
46 + n > 63
n > 63 - 46
n > 17

Therefore, at least 18 items Homeroom B needs to collect.