arithmetic series - solving problem

Question

I don't understand this question.

The 10th organ is 4 times bigger than the 3rd organ. The amount of all 50 organs together is 3725.

Find D and a1.

How can I solve that? I have absolutely no way.

Answer 1

HINT:

Do you know if $a_1,D$ be the first term & the common difference respectively,

$n$th term $=a_1+(n-1)D$

Sum of $n$ terms $\displaystyle=\frac n2\{2a_1+(n-1)D\}$

So, you will have two equations with two unknowns

Reference : Arithmetic progression

Answer 2

Hint: Assuming the organs are in arithmetic progression in size, the size of organ $n$ is $a_1+d(n-1)$ Now plug in what you are given: $a_1+9d=4(a_1+2d), 50a_1+\frac 12(49)(50)d=3725$. Two equations, two unknowns.