Explicação passo-a-passo:
50 - 3 × {7+8 : 4 - [ 20 - 3 × (9 - 5)]}
= 50 − 3( 7 + 2 − (20 − 3(9 − 5)))
= 50 − 3( 9 − ( 20 − 3( 9 − 5 )))
= 50 − 3( 9 − ( 20 − (3)(4)))
= 50 − 3( 9 − (20 − 12))
= 50 − 3(9 − 8)
= 50 − (3)(1)
= 50 − 3
= 47
