We want 4/5 of a number to be 144. This means that

Multiplying both sides by 5/4, we have

In order for the product of some numbers to end in a zero, the prime factors of ... So 1 x 2 x 3 x ... x 98 x 99 x 100 (AKA 100!) has 24 zeroes at the end
7(x-2) +3x=46
7x-14+3x=46
10x-14=46
10x=60
X=6