Question

(1+2+3+...+50) × a - 123,4=3701,6

Answer 1

1+2+...+50=(50•51)/2=1275
1275a-123,4=3701,6=>1275a=3825/(:25)=>51a=153=>a=3

Answer 2

(1+2+3+...+50) × a - 123,4=3701,6[50*(50+1)]/2  × a=3701,6+123,4
(50*51)/2  × a=3825
25*51  × a=3825
1275 × a=3825
a=3825:1275
a=3