Mars bought 120 pens for his classmates. 40% of them are black. 37.5% of the color pens are red and 29% are green. If the remaining are yellow, what percent of pens are yellow?

40% of 120=48 black, so 120-48=72 are coloured. 37.5% (3/8) of 72=27 red, 29% of 72=0.29*72=21 green approx. So red+green=48 and 72-48=24 yellow. 24/120=1/5=20% of the 120 pens are yellow.

We donâ€™t actually need to know there were 120 pens to work out the percentages. If 40% were black then 60% were coloured. 37.5% of these=22.5% red, and 29% of 60%=17.4% green. So 22.5+17.4=39.9%. That leaves 60-39.9=20.1% yellow.

The first way of working out the answer is easier to visualise because it deals with actual numbers of pens.

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