From the ukuleles, there are 2 * 4 = <<2*4=8>>8 strings
From the guitars, there are 4 * 6 = <<4*6=24>>24 strings
From the violins, there are 40 - 8 - 24 = <<40-8-24=8>>8 strings
The total number of violins Francis' family has is 8 / 4 = <<8/4=2>>2 violins
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