Suggested answer:

Common multiple of $9 and $12 will be $36

If both spent $36 each, J bought 8 large, K bought 21. Therefore, K have 13 more muffin

Total difference => 156

156 / 13 = 12 group of $36 each.

$36 x 12 = $432

Jim spent $432 in total.

Check

$432 can buy (432/9) × 2 = 96 large muffins

$432 can also buy (432/12)  × 7 = 252 small muffins

252 – 96 = 156

Checked