A shop only sold black, grey and white shirts. 7/12 of the shirts
12 in the shop were white and 56 shirts were grey. If 1/3 of the
shirts were black, how many shirts were there in the shop?
I will assume you have a typo and you meant:
"7/12 of the shirts in the shop were white"
total number of shirts --- x
7/12 x + 56 + 1/3 x = x
multiply each term by 12, the LCD
7x + 672 + 4x = 12x
x = 672
state your conclusion