in c1: =A1*B1
in c2: =IF($A2=0,($C1/$B1)*$B2,$A2*$B2)

Copy down

=IF($A2=0,$A1*$B2,$A2*$B2) does work.

However, when it reaches areas with the triple merge
e.g., (A3/A4/A5) to equal A3*Bx
there are two problems. Copy down puts A5*B5 in C5, and the IF only looks
at two cells. Perhaps the IF could be nested and handle, say, up to 5
merged, but copy-down would still fail...

Thanks, though!