Răspuns: a = 1
Explicație pas cu pas:
[ ( 15 × 6 - 10 × 3 × a ) × 4 + 15 × 4 ] × 10 = 30 × 100
[ ( 90 - 30 × a ) × 4 + 60 ] × 10 = 3 000
( 90 - 30 × a ) × 4 + 60 = 3 000 : 10
( 90 - 30 × a ) × 4 + 60 = 300
( 90 - 30 × a ) × 4 = 300 - 60
( 90 - 30 × a ) × 4 = 240
90 - 30 × a = 240 : 4
90 - 30 × a = 60
30 × a = 90 - 60
30 × a = 30
a = 30 : 30
a = 1 → numarul, aflat prin metoda mersului invers