He has 1/4 of his garden to plant 3 types of onions equally in that space. Divide 1/4 by 3.
= 1/4 ÷ 3
when dividing fractions, multiply by reciprocal/inverse
= 1/4 ÷ 3/1
= 1/4 * 1/3
= (1*1)/(4*3)
= 1/12
He has 1/12 of his garden for each type of onion.
Hope this helps! :)