"Is" definitely modifies "one computer". But I think sentence is bit ambiguous: "what does four times as many as.." modify? Is it referring to the improvement of the situation in which more computers are shared among fewer people, or worsening the situation in which people are four times to share one computer, implying less computers among more people. In any case, "were" sounds better in the sentence in comparison to the closest alternative "was". Could this be because it is an hypothetical situation?
Yes, one microcomputer "is"...
The way to read the "four times as many as there were" part is to read the first part as:
"one micro computer for every 32 pupils"translate to: 1 out of 32
"four times as many as there were"translate to: 1/32 = 4 times (some number)
In other words, the sentence is implying the ratio was 1 / (32*4) = 1 / (128) four years ago.No need to do the math, of course--but that's basically what the sentence is implying.
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