When x = 10 minutes, then y = 4 grams.  
When x = 50 minutes, then y = 15 grams.

This information can be interpreted as two points on a line.
So the problem is the same as:

Find the equation of the line passing through the points (10,4) and (50,15)

We use the slope formula:



We use the point-slope formula:



Substituting:



Multiply through by 40









Add 160 to both sides:



Divide through by 40





Or you could make the fractions into decimals:



Edwin