First add the ratios together: 2+3+7=12 then divide £600 by 12=£50. So now we can split the amount into 50s, so 2×50=£100, 3×50=£150, 7×50=£350, Carl's original share.
If Carl were to receive £140 instead of £350, this is 140/350=2/5 of £350. All amounts, including the total will be reduced by this amount. Ann gets ⅖×100=£40, Bob ⅖×150=£60. Total=⅖×600=£240=40+60+140.