3 large beads cost 2.50 * 3 = 7.50
30-7.50 = 22.50
20-7.50 = 12.50
so they can spend between 12.50 and 22.50 on small beads
12.50 / 1.25 = 10
22.50/1.25 = 18
can buy between 10 and 18 small beads
1.25x >=12.50 and 1.25x<=22.50
