Model Drawing Would Save The Day

I saw a fb post by a father asking for help with his child's year 4 maths homework. It is an interesting question, requiring a degree of analysis that I would classify as problem solving.

The question:

Mr Lee had 280 red and white T-shirts for sale.
After selling 109 white T-shirts and 1/5 of the red T-shirts, he had an equal number of red and white T-shirts left.
How many red T-shirts did he have for sale at first?

To solve this problem, the child would probably start with this:

280 - 109 = 171

Then what's next?

A common error for this type of problem would be to divide 171 by 8, thus getting an answer of 21.375, which we would immediately know is wrong because we can't have a 0.375 T-shirt. 

In a problem solving situation like this, drawing a model would help the child analyse the problem and derive at the correct answer. The next step should be 171 divide by 9. The last step would be...?

Would you like to try drawing the model and find the answer for yourself?