402 + [ 1002 : 3 - ( 100 + a ) × 2 ] : 4 = 408
402 + [ 334 - ( 100 + a ) × 2 ] : 4 = 408
[ 334 - ( 100 + a ) × 2 ] : 4 = 408 - 402
[ 334 - ( 100 + a ) × 2 ] : 4 = 6
334 - ( 100 + a ) × 2 = 6 × 4
334 - ( 100 + a ) × 2 = 24
( 100 + a ) × 2 = 334 - 24
( 100 + a ) × 2 = 310
100 + a = 310 : 2
100 + a = 155
a = 155 - 100
a = 55