Question 978921
The nth term of a geometric sequence can be written a_n = a + (n-1)d, where a is the 1st term and d is the common difference
So we can write the 1st 4 terms as 
a_1 = a
a_2 = a + d
a_3 = a + 2d
a_4 = a + 3d
Given: a_2 = 50, a_4 = 112.5
subtract a_2 from a_4 to determine d:
a + 3d - (a + d) = 112.5 - 50
2d = 62.5
d = 31.25