Folosesti formula S=[(a1+an)*n]/2
Notam S = 1+2+3+...+49 = [(1+49)*49]/2 = (50*49)/2 = 25*49 = 1225
S/105 = 7/x
1225/105 = 7/x
1225x = 105*7
1225x = 735
x = 735/1225
x= 21/35 = 3/5