Explain how modeling partial products can be used to find the products of greater numbers.

It is easier to multiply. For example: 63 multiply by 7
63 x 7 = (60+3) x 7 =  60 x 7 + 3 x 7 =  420 + 21  =  441

Another example : 25 x 73
25 x 73 = (20+5) x (70+3) = 20 x 70 + 20x3 + 5x70 + 5x3

                                         = 1400 + 60 + 350 + 15           =  1875

Let, Fiona's age = x
Darwin's age = 2x+6

Then, 2x+6 = 50
2x = 44
x = 22

So, your answer is 22

Hope this helps!