Answer:
40 pens
Step-by-step explanation:
Let the total number of pens Ron had be x.
We need to find the fraction of the pen that were blue.
3/8 of them were black and 1/4 of them were red. This means that the fraction of the pens that were blue is:
1 - (3/8 + 1/4) = 1 - 5/8 = 3/8
There were 15 blue pens. This means that 3/8 of x is 15:
3/8 * x = 15
x = 15 * 8/3 = 40 pens
Ron had 40 pens.