Mrs Lao bought 420 blue pens and 160 red pens. She sold some of the red pens. After that, 7/9 of the pens left were blue pens. How many red

Let's call the number of red pens sold = S   so...we have

The number left x (7/9)  = 420...or, mathematically.....

[420 + 160 - S] (7/9) = 420     multiply both sides by (9/7)

[420 + 160 - S]  = 540

580 - S  = 540       rearrange

580 - 540 =  S  = 40   red pens sold