a-b = 640
a : b = 4 rest 13
a = 4×b+13
a |___|___|___|___|+13
b |___|
4 segmente egale +13 - 1 segment egal = 3 segmente egale +13 = 640
640 - 13 = 627
637 : 3 = 209 ( un segment egal)
a = 4×209+13 = 836+13 = 849
b = 209
Verificare:
849 - 209 = 640
a = 849
b = 209