Compatible numbers are numbers that seem to "go together" because they are easy to compute mentally. The use of compatible numbers is an estimation strategy.
Bob bought a package of 4 pairs of athletic socks for $10.99. About how much did he pay for each pair?
$10.99 is close to $12.00 (12 is a multiple of 4).
12 ÷ 4 = 3
Thus, each pair of socks cost about $3.00.
See also estimation strategies.